I'm going to write out all of the money he gets each week, and when he has enough to pay back his sister, I will subtract the money he payed her.
35 5 40 75 110 145 180 215 250 285 220 255 290 325 360 395 430 465 500 535
Now count how many numbers there are (20), and that's your answer! :)
The answer is 20
Hopefully this helps! If you have any more questions or don't understand, feel free to DM me, and I'll get back to you ASAP! :)