CareerCup

Yes, the answer seems to be (24^2)+(23^2)+....+1 = 4900

let's say N balls in base.
so from 625 balls we need to make pyramid.
hence , n+(n-1)+....+1=625.
this equation will get solve by positive integers id if we add 25 balls from bag to those 625balls alredy given....25 is least no balls need to add to get postivi integer solution.

structural analysis? I can construct a pyramid using just three balls. why do I need 625 balls??? What did the interview want exactly???

I guess the base level has 625 balls. The second to the base level has 624, and so on so forth. So, the total number is,

(625 +1) * 625/2.

Are there any tricks here?

I think, we want to build a pyramid of 625 balls....So, n = 625,
how many more balls will be required ie balls from bag to build the pyramid
So, ans = 624*625/2

Assuming its a square pyramid and base level has 25X25 balls
then the n-1 level has 24X24 balls...
so seeing this progression n^2 + (n-1)^2 + ...... 1
n(n+1)(2n+1)/6 = 25*4*51 = 5100 . So required balls are 5100-625 = xxx

Answer is n^2 ! where n = 25

Base is 25 x 25
Next layer is 24 x 24
Till you get 1 on the top

625 balls are already there on the ground. (Level 0)

624 balls in Level 1, 623 in Level 2.......1 in Level 625.

Hence, the answer is 624*625/2 more balls.

Chintan is right

Hey, The answer is as follows:
(25^2)+(24^2)+(23^2)+......(1^2)

the n^2 series is correct....

hi everyone i think it says we have to make pyramid out of 625 balls
so if base have 16 balls next layer havce 9 balls in square pyramid and next have 4 balls so on while caculating this number 1+4+9+16+25+49+64+81+100+121 it gives sum equal to 506 so remaining is 625-506=119

I agree with Ravikant but I suppose there is some calculation mistake.
You need 1 + 2^2 + 3^2...12^2 balls. The sum comes out to be: 650
Since we already have 625 balls in hand, we just need to take 25 more balls from the bag.

Here's the answer
(25^2)+(24^2)+(23^2)+......(1^2)

http://z.hubpages.com/u/109007_f520.jpg

In other words (n^2)!

Hey pyramid is 3 d structure for a pyramind of height 1, 1 ball is required, height 2 1+3 = 4 balls required height 3 1+3+6 balls are required and so on....

A pyramid of height n requires n(n+1)(2n+1)/6 balls, find the closest value greater tha 625, which happens to be 680 for n=15...Therefore 55 more balls are required

Since pyramid is 3D, After the 25x25, don't you think the next one should be 23x23...
25^2 + 23^2 + ..... + 1^2
Correct me if my idea is wrong.

You're all trying to leap into solving the puzzle without so much as a mention of the fact that the "structural analysis issues" part is the most important part of the question.

Without an effective answer to this second question you cannot possibly produce a meaningful answer to the first part.

Meanwhile, Microsoft, this is a lame question to throw into an programmer interview, if that's what it is. Arbitrary IQ problems will not decide who the best programmers are, real-world software development issues are what you want to see solved. There is very little point in analyzing logical thinking beyond the context of actual developmental issues.

You're all trying to leap into solving the puzzle without so much as a mention of the fact that the "structural analysis issues" part is the most important part of the question.

Without an effective answer to this second question you cannot possibly produce a meaningful answer to the first part.

Meanwhile, Microsoft, this is a lame question to throw into an programmer interview, if that's what it is. Arbitrary IQ problems will not decide who the best programmers are, real-world software development issues are what you want to see solved. There is very little point in analyzing logical thinking beyond the context of actual developmental issues.

We can have answer as 15*10*7*3*1=3150 balls ... so we need extra balls from = 3150-625=2525