Hi andi01,

State what you know and what you don't know:

Let x = the number of tires installed

Let 40 = total number of installations

Let 40 - x = the number of gears he assembles

Let 2x = earnings on tire assemblies

Let 5(40 - x) = earnings on gear assemblies

Let 110 = total earnings

Use the last three statements to set up your equation.

2x + 5(40 - x) = 110

I think you might be able to take it from here.