harshavardhanc wrote:

gmatmachoman wrote:

harshavardhanc wrote:

sanju09 wrote:If two pipes A and B together can fill a cistern in x minutes and if A alone can fill it in a minutes more than x minutes and B alone can fill it in b minutes more than x minutes, then what is x equal to?
(A) √ (a^2 + b^2)
(B) √ (a^2 - b^2)
(C) a b
(D) √ (a b)
(E) a b - a^2 - b^2

since this relation is a generic one, it should be satisfied by all the values.

Let's take equal and simple values, and then go about it.

let A and B each fill the cistern in 4 mins. therefore, together they will take half the time , i.e. 2 mins to fill the cistern.

according to the question X= 2, a=2, b=2

put the values, check for correctness.

heheheh!! U logic wont work bro....Try for other values of X where a is not equal to b...

Only the straight method wrks as posted by @truplayer256.

Gmatmachoman..buddy, I know you are very experienced on this forum and I think you have a strong prep too.

But, this CR has made me so adamant that I cannot take a plain no. I need solid reasoning for any argument.

now coming to my previous response, there is NO logic involved in my first post. None at all.

it's like saying : if you have an equation of line as X+Y = 2, sum of every set of value on this line will be 2 . Simple.

Now let me show you how this is true. I'll take some difficult numbers, calculation verification for which is left to you .

Let A take 4 mins to fill the cistern and B take 5 mins. So working together, they well take 20/9 mins to fill the cistern

Hence, per the question 'a' becomes 4-(20/9) = (16/9) and 'b' becomes 5-(20/9) = 25/9

therefore X = 20/9 = sqrt( 16/9 * 25/9)

no option other than D will give you the correct answer.

plugging in the values ALWAYS works. Sometimes it gives two probable answers. You then need to take another set which eliminates the wrong choice.