Math question really stuck

[Ghost Of Highbury]

heres d question : 2 pipes A and B can fill a tank in 36 hours and 45 hours respectively. If both pipes are opened simultaneously, how much time will be taken to fill the tank?

[Ghost Of Highbury]

anyone dere..

nid404..plz help..!!

[astarmathsandphysics]

I cant find this question where is it?

[Ghost Of Highbury]

i got it from a practice book...

[godfather93]

there is no other hint?

[O.T.13.]

i think this is average
just add them and divide them by two, can't think of anything else, and there aint that much numbers to work with

[Ghost Of Highbury]

the answer is 20...don't know how they solved it,,,

[astarmathsandphysics]

heres d question : 2 pipes A and B can fill a tank in 36 hours and 45 hours respectively. If both pipes are opened simultaneously, how much time will be taken to fill the tank?

flow rate for 1 =f1 and for 2=f2  so 36f1=V and 45f2=V so f1=V/36 and f2=V/45 and total flow rate =f1+f2=V/36+V/45=81V/1620=V/20

so f=V/20 and 20f=V and time =20 hours

[Ghost Of Highbury]

no more hints given,,,

[Ghost Of Highbury]

okk..
thanks a lot astar..
really good explanation