Hi there!
Let the amount of weeks be represented by X.
Thomas earns $20
every week,
so has a total earning of $20
times the amount of weeks, hence 20X. Now we only need to add the additional $800 to find the correct expression.

Thomas earns $30
every week, so has a total earning of $30
times the amount of weeks, hence 30X. Now we only need to add the additional $600 to find the correct expression.

Now we can set up and solve the expression.

Subtract 800 from both sides.

Subtract 30x from both sides.

Divide both sides by -10.

Therefore, in 20 weeks the earnings will be the same. To find the exact amount of earnings we have to plug in x = 20 into one of the expression.

Hence, the earnings will be $1200.
~Hope this helps you!