PIPE RATE (TANKSWORTH/HOURS) TIME VOLUME
First 1/10 3&1/2 (1/10)(3&1/2)
Second 1/x
Combined 1/10+1/x 4 4((1/10)+1/x)
Examine the tabulated description for a few minutes.
First pipe filled some of the tank volume during 3&1/2 hours and then BOTH pipes continued to fill the rest of the tank's volume for 4 hours. This was filling of the whole 1 tank volume.