An inlet pipe on a swimming pool can be used to fill the pool in 1818 hours. the drain pipe can be used to empty the pool in 3636 hours. if the pool is 2323 filled and then the inlet pipe and drain pipe are opened, how long from that time will it take to fill the pool

The rate of the inlet pipe is 1/18 of the pool volume per hour while the drain pipe is 1/36 volume per hour.
The net rate meaning if the two pipes work simultaneously is 1/18 – 1/36 = 1/36 of the pool volume per hour.The equation to know the t for filling 1/3 of the remaining volume of the pool is 1/36 * t = 1/3Solving for t is through cross multiplication. Thus it gives us t = 12.

Ithink the answer would be a 

The density of the block of iron is lees than the density of mercury but the density of the block of iron is greater than the density of water