Question 204760
weight of juice content only = J
weight of an empty juice box = E
weight of a grape = G


1st equation:
2J + 2E = 18G + 1/2 J + E
2J - 1/2 J - 18G + 2E - E = 0
4/2 J - 1/2 J - 18G + E = 0
3/2 J - 18G + E = 0


2nd equation:
E = 6G


substitute E = 6G into the 1st equation:
3/2 J - 18G + E = 0
3/2 J - 18G + 6G = 0
3/2 J - 12G = 0
3/2 J = 12G
J = 12G * 2/3 = 24/3 G = 8G


so, a full juice box equals with J + E = 8G + 6G = 14G = 14 grapes
the juice content only  (without the box) equals with J = 8G = 8 grapes