40 - x = 25
40 - 25 = x
x = 15
x + 28 = 71
x = 71 - 28
x = 43
x - 31 = 49
x = 49 + 31
x = 80
34 + x = 62
x = 62 - 34
x = 28
x + 29 = 74
x = 74 - 29
x = 45
100 - x = 37
100 - 37 = x
x = 63
40 - x = 25
40 - 25 = x
x = 15
x + 28 = 71
x = 71 - 28
x = 43
x - 31 = 49
x = 49 + 31
x = 80
34 + x = 62
x = 62 - 34
x = 28
x + 29 = 74
x = 74 - 29
x = 45
100 - x = 37
100 - 37 = x
x = 63