Basically a distance multiplied by a weight that is equal to the distance that is going to be multiplied by the weight. (for the equation we will use X for the distance).
equation: 4 x1 20 = ? x 80
Now step one: 4(120) = X(80)
Or another way is 480 = 80X
480/80 = X
48/8= X
X = 6
I hope this could help! Sorry if it didn't make much sense otherwise!