Let the number of away fans = x
Then, the number of home team fans is 2x - 31
So
x + ( 2x - 31) = 86
3x - 31 = 86 add 31 to both sides
3x = 117 divide by 3 on both sides
x = 39 and that's the number of away fans
And
2(39) - 31 = 78 - 31 = 47 and that's the number of home fans
