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?
In the beginning, Martha lost 18 / 2 = <<18/2=9>>9 crayons.
So after she bought a set of new crayons, she has in total 9 + 20 = <<9+20=29>>29 crayons.
#### 29