Martha has 18 crayons. She lost half of them, so she bought a new set of 20 crayons. How many crayons in total does Martha have after the purchase? Let's be accurate as possible.
In the beginning, Martha lost 18 / 2 = 9 crayons. So after she bought a set of new crayons, she has in total 9 + 20 = 29 crayons.
The answer: 29.