12. Ainsley has decided to begin a 5-day sit-up challenge where she plans to increase the number of sit-ups completed each consecutive day by the same amount to
gradually build up her core muscle strength.
After talking with a friend who already began the challenge, she's aware that she needs to complete 20 sit-ups on Day 1 of the challenge and will complete 36 sit-
ups on the last day, Day 5.