One inlet pipe can fill an empty pool in 9 hours, and a drain can empty the pool in 15 hours. how long will it take the pipe to fill the pool if the drain is left open?

For every 1 liter in there is 9/15 liters out
effectively the influx is 4/15 liters

Proportion
1 liter / 9 hours = 4/15 liters / X hours
X = (4/15 liters * 9 hours) / 1 liter
X = 36 hours

Idon’t exactly understand but i think dbc=rst