what is to be subtracted from 7/15 to get -4/5

what i did not understand the question
 

SOL. let the number to be subtracted be X

        therefore 7/15-X=-4/5
                                X=7/15-(-4/5)
                               X=17/15=4/5
                               X=19/15

Hence the number to be subtracted from 7/15 to get -4/5 is 19/15