Math task for kids - dare to solve it? :)

[just a punk]

My son (he is 13 BTW), just asked me to help him with this school math task:

What is a sum of figures (digits) in the result of the simple expression below?
2^2008 * 5^2011

I've solved it with my son already, but will someone here able to solve it? Remember it's a just a homework task for 13 y.o. kids

P.S. "^" in that expression is a power sign (to raise a given number to the given power)

[just a punk]

Hmm.. 21 views and no single try to solve it? C'mon guys, it's just a standard school task for kids, but not something from a high math course!

[just a punk]

By sum of figures, I meant the following:
124 = 1+2+3 = 6
10082 = 1+0+0+8+2 = 11
7 = 7 = 7
82637 = 8+2+6+3+7 = 26
etc..

[quantum-x]

Answer is 8

[ShaveBucks]

Originally Posted by cyberxxx

By sum of figures, I meant the following:
124 = 1+2+3 = 6

If I was your son, I'd never ask for your help again

[just a punk]

Originally Posted by ShaveBucks

If I was your son, I'd never ask for your help again

Just a typo of course

[just a punk]

Originally Posted by quantum-x

Answer is 8

The answer is accepted. Anyone else?

I will tell which one is right and will show the calculation if necessary, but just let's wait for more answers. Don't post your calculations please, just the answers. Otherwise it won't be interesting.

[quantum-x]

Originally Posted by cyberxxx

The answer is accepted. Anyone else?

I will tell which one is right and will show the calculation if necessary, but just let's wait for more answers. Don't post your calculations please, just the answers. Otherwise it won't be interesting.

zzzzz - the answer is 8.
You can even do it in windows calculator if you're lazy.

[woj]

The answer is 12500....0000 so 8, come on, not very hard...

[Eva PSC]

Originally Posted by woj

The answer is 12500....0000 so 8, come on, not very hard...

this is one crazy pic in your avatar, my eyes hurt to look at it while trying to figure it out lol

nice shop job

[woj]

It looks tricky, but a quick flashback to grade 6 math, and you get:

2^2008 * 5^2011 = (2^2008)*(5^2008)*(5^3)=125*(10^2008)

[L-Pink]

Originally Posted by ShaveBucks

If I was your son, I'd never ask for your help again

[D]

Why would we keep it going after the first answer was the right one?

[just a punk]

Originally Posted by woj

It looks tricky, but a quick flashback to grade 6 math, and you get:

2^2008 * 5^2011 = (2^2008)*(5^2008)*(5^3)=125*(10^2008)

Congrats! This is the only right answer. Windows calculator is not an option, because you have to prove the solution. In a way as woj did above.

[quantum-x]

Originally Posted by cyberxxx

Congrats! This is the only right answer. Windows calculator is not an option, because you have to prove the solution. In a way as woj did above.

As I said, you can *also* do it in calculator.

[just a punk]

Originally Posted by D

Why would we keep it going after the first answer was the right one?

Because after that, the one should be explaining on how he got it. As I said, Windows calc is not a right way to do that (you won't have it on a school's exam)

[tranza]

Boring.......

[ADL Colin]

Originally Posted by D

Why would we keep it going after the first answer was the right one?

to look for wrong ones

[just a punk]

Originally Posted by quantum-x

As I said, you can *also* do it in calculator.

Sorry, I've missed that *also*. Course if you did it manually, your answer is also correct.

[just a punk]

Originally Posted by woj

The answer is 12500....0000 so 8, come on, not very hard...

Ok, here is another one from a same today's homework.

1*2*3*...*n = 2^15*3^6*5^3*7^2*11*13

What is n there?

[Tubestone]

1+1+1+1+1+1+1+1= 8

[munki]

The answer is 42

[woj]

Originally Posted by cyberxxx

Ok, here is another one from a same today's homework.

1*2*3*...*n = 2^15*3^6*5^3*7^2*11*13

What is n there?

16 ....

[just a punk]

Originally Posted by munki

The answer is 42

Course it's a wrong answer BTW, it's ok to post your calculations to show it wasn't solved with a windows calc

[D]

Originally Posted by cyberxxx

Ok, here is another one from a same today's homework.

1*2*3*...*n = 2^15*3^6*5^3*7^2*11*13

What is n there?

n = 16

[D Ghost]

math is for calculators

[munki]

Originally Posted by cyberxxx

Course it's a wrong answer BTW, it's ok to post your calculations to show it wasn't solved with a windows calc

No it's not...

The answer is always 42...

[aico]

Originally Posted by ShaveBucks

If I was your son, I'd never ask for your help again

[just a punk]

Still no one is able to prove the answer with formula?

[quantum-x]

Originally Posted by cyberxxx

Still no one is able to prove the answer with formula?

We're bored of this already.

[D]

Originally Posted by cyberxxx

Still no one is able to prove the answer with formula?

We wrote the answer, already. :-P

It's just a matter of expanding the right of the equality and then rewriting it in factorial form.

2^15*3^6*5^3*7^2*11*13=

2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x3x3x3x3x3x3x5x5x5x7x 7x11x13=

2x3x(2x2)x5x(2x3)x7x(2x2x2)x(3x3)x(2x5)x11x(2x2x3) x13x(2x7)x(3x5)x(2x2x2x2)=

2x3x4x5x6x7x8x9x10x11x12x13x14x15x16=

16 factorial=

16x15x14x13x12x11x10x9x8x7x6x5x4x3x2x1=

1x2x3x4x5x6x7x8x9x10x11x12x13x14x15x16

n=16

Q.E.D.

Still... good to see a father taking such an interest in his son's math homework. If more did so, I think we'd be further along a better path.

[just a punk]

Originally Posted by D

Q.E.D.

Very impressive! I know nothing about your math background but it's much above the average webmaster here, because I mentioned the number of topic views but only D gave a right answer to the second task.

Originally Posted by D

Still... good to see a father taking such an interest in his son's math homework. If more did so, I think we'd be further along a better path.

Thanks, I'm just to see him growing smart. I believe I do like every other parent.