B the water injection per hour is y.

Half the time to fill the pool is set to t hours.

A single injection of a tube can fill the pool within 5 hours; The second test tube can fill the pool in 10 hour. The pool is full at this time, which means 5x= 10y.

Then x=2y

(x+y)t=5y When two pipelines are injected at the same time, (2y+y)t=5y.

T=5/3 hours