Write a real world problem that correspond to the equation 10x + 30 =100?

10x + 30 = 100

Here is a real world problem to demonstrate this. 

Mack decides to burn some calories, so he attends a gym. A cost of a gym costs 10 dollars per month, with an addition registration fee of 30 dollars. Mack has spent 100 dollars on the gym so far. How many months has he been participating at the gym? 

Let 'x' represent the amount of months spent. 

This is too long-standing now get out