The **Understanding 2-digit numbers** exercise appeared under the Early math Math Mission. This exercise helps users to practice adding numbers less than ten to numbers from 1 to 90. It maintains an emphasis on grouping by tens.

## Types of problems

There are three types of problems that show up in this exercise:

- Fill in number above: The problem has a grid with a circled number. It asks the user to identify and write in a number that is some number of spots less than ten beyond the given number.
- Split a number: This problem asks users to perform a multiple selection problem where they are to select all answer choices that correctly split a number into groups of ones and tens.
- Find name of number by grouping: This problem has a collection of objects split into a number of rows of ten and an additional row that has less than ten. The user is to identify how many rows of ten there are, how many additional objects there are, and finally how many objects there are in total.

## Strategies

Some of these problems run very smoothly but the multiple select can cause some slowing down because of the varied use of tens and ones. This exercise is easy to get accuracy badges, and medium difficulty for getting speed badges. The "tab" button can help button can make these problems faster.

- On the third type of problem, since numbers between ten and 100 are named by their tens and their ones the entries all relate nicely. For example, if the there are 2 rows of ten and 3 extra, the correct entries for the slots are 2, 3, 23.