RATES
Large pipe 1/x
Small pipe 1/(x+4)
Both pipes 1/x+1/(x+4)
RT=V rule for filling rates and volume.
You can assume the amount of volume to be 1 POOL for this fill rates exercise.
Three hours both pipes, and then 2 hour for just small pipe to finish the 1 pool.
Some algebra steps, but just solve for x.