Answer: 6
Work Shown:
He eats ten 12-calorie crackers, so that is 10*12 = 120 calories total
If x is the number of 20-calorie cookies, then the total number of calories here is 20x. We want this to be 120, so,
20x = 120
x = 120/20 .... divide both sides by 20
x = 6
He must eat six 20-calorie cookies to get 120 calories total