4 5 y 7 4 21 8 July 8, 2019 16 32 64 128 4 5 y 7 4 chia 21 8 (4 5-y)•5 8=8 7 4 5(-8y-7)-3 9(6-9y) 4 7 - 2 5 (y + 4 5) / 7 =1 2