Question

why is it less effective to wash an insoluble precipitate with 15 ml of water once than it is to wash the precipitate with 3 ml of water 5 times ?

Answer 1

saying a precipitate is "insoluble" doesn't mean that even a small amount won't dissolve in a large volume of water. If you wash a precipitate with lots of water, chances are you'll force a tiny bit to dissolve, and pass through the filter. If you wash it with less water, but more often, then less is allowed to dissolve each time, so you'll lose less mass through the filter (probably)