the tank is 3/4 filled.
3/4+5 gallons=7/8 part
so 5 gallons =7/8-3/4=1/8 part of the tank

so the capacity of the tank = 5/(1/8)= 40 Gallons

So the choice is B

It's 40

Here is how to solve.
3/4=6/8
6/8+5=7/8
so we now know that 1/8=5galons
5*8=40

So our answer is B

i tried backsolving. atleast for these kinda sums it helps me a lot

Backsolving is a good method to solve. Yes. But it's better to get hold of algebraic approach as well. On the test day, it's good to know both approaches. If you don't get it right with one approach, you could always take the other approach.

bhumika.k.shah wrote:i tried backsolving. atleast for these kinda sums it helps me a lot

its just a matter of framing eqn ; backsolving indeed helps but if the ans is E then trying with 4 ans will kill time
for this one eqn is

3t/8+5=7t/8

even if u r trying to do backsolving always try with otion C(the middle one) and based upon the resultant decide whether we need to go up or down ( choices in gmat are always in inc or dec order) , with such an approach we can save a lot of time

yes thephoenix. when it comes to back solving i always start with option C and then choose other options accordingly.
Thanks!