5 - 2 = ?
3 - 1 = ?
9 - 5 = ?
6 - 4 = ?
7 - 3 = ?
5 - 5 = ?
4 - 1 = ?
3 - 2 = ?
4 - 2 = ?
6 - 3 = ?
8 - 4 = ?
1 - 1 = ?

Exercise(s)

The obesity problem can be resolved through exercises and calorie limitation.

Exercise is one of those words that can be countable or uncountable, depending on what you mean!

Exercise to stay healthy

Here you need the uncountable form of exercise:

The obesity problem can be resolved through exercise and calorie limitation.
maths exercise
jogging is good exercise (uncountable) for losing weight and keeping fit

Exercises to improve ability (including physical ability!)

Here you need the countable form of exercise (don't forget to add s if plural!):

Polynom is an app that helps students to get higher scores by solving and explaining math exercises.
maths exercise
doing maths exercises (countable)
Stretching exercises help to strengthen muscles and improve overall strength and agility.
maths exercise
stretching exercises (countable)