This is an arithmetic sequence because it increases by a constant amount each week. To start, you need to write an arithmetic function that goes with this, which would look like this:
f(x) = starting value + (x-1) * common difference
This is basically saying that you started with 20 and add 10 dollars every week
When you plug in your values, you get this:
f(x) = 20 + (10-1) * 10
Then, you solve it to get your answer.
f(10) = 20 + (10-1) * 10
f(10) = 20 + 9 * 10
f(10) = 20 + 90
f(10) = 110
I hope this helped!