If the hypotenuse is a multiple of 5 and another length is the same multiple of 3 or 4, the one left over will be the same multiple of the one that's left.
you want to find the mass of only the grapes, not including the bowl. so in this case, you would subtract the mass of the bowl from the total mass. you do this because you dont need the mass of the bowl. 256g - 121g = 135g :)