中4 maths

suppose the volume of the pool is A,
then for one hour, smaller pipe could fill volume of A/6 (from the 3rd sentence of the question)
and for one hour, both pipes could fill volume of A/4(from 1st sentence of the question)
therefore big pipe, in an hour, could fill volume of:
a/4-a/6
=a/12
it fill 1/12 of the pool for one hour, that is, it takes 12 hours to fill up the pool.

By the way, the "smaller pipe" should be larger than the "larger pipe"!!!!!!!!!
Therefore, if we are asked "find the time it would take the larger pipe to fill up the swimming pool." then we should say "6 hours"

good!

Let A hours to fill up the pool by larger pipe

So, take 6+A hours to fill up the pool by small pipe

Fill up 2 no. of swimming pool to be A + (6+A) =4 X2
so, 2A+6=8

Therefore, A=1

One hour to be taken for filling swimming pool by larger pipe