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