Water leaks from a crack in a vase a rate of .5 in.³ per minute how long does it take for 20% of the water to leak from a full vase

If the volume of the vase is V, then 20% of the water in the vase, represented by 0.20V, leaks out at the rate of 0.5 in^3.  Our job is to calculate how long this takes.
                                                                      quantity
Quantity = rate times time, so that (time) =
                                                                        rate
                                                                                                  0.20V
Then (time required for 20% of the water to leak out) =
                                                                                         0.5 in^3  per min

                                                                        = 0.4V (we don't know V)

The time req'd for all of the water to leak out from a full vase is 

                                                                 (time) = 5(0.4V) = 2V minutes