Alternate workouts?

Kaitlyn • I`m 19 and NTTC
Ive started this 30 day ab workout and every day it gradually increases the reps. One of the exercises is doing sit ups. Now 10-15  sit ups is fine but I'm having to do 40+ daily and it's hurting my back! Is there any alternate exersice that works the same muscles as a sit up but is less strenuous on the back?