You can use this app for calculate either direct or inverse proportions.
Directly proportional: as one amount increases, another amount increases at the same rate.
If you work 2 hours you get paid $40
If you work 3 hours you get paid $60
x = (c * b) /a
Inversely Proportional: when one value decreases at the same rate that the other increases.
As speed goes up, travel time goes down
x = (a * c) / b