Subtract within 20

The first instance of the Subtraction within 20 exercise is under the Early Math Math Mission. This exercise practices basic subtraction facts using numbers up to twenty.

Types of problems
There is one type of problem that shows up in this exercise:
 * 1) Subtract the numbers: A subtraction problem is given and the student is asked to provide the answer.Subin20.png

Strategies
This problem is easy to get both accuracy and speed badges since the problem involved has fairly small numbers and there is question in each problem.