Skill: Solve part-part-whole problems

A part-part-whole problem has:

• two parts that make
• one whole.

First, find out whether the missing number is a part or the whole.

Example:
A library has 27 books. 12 books are picture books. The rest are storybooks. How many storybooks are there?

The whole is 27 books. One part is 12 picture books. The other part is unknown.

You can show this with a bar model:

$$\boxed{12\text{ picture books}}+\boxed{?\text{ storybooks}}=\boxed{27\text{ books}}$$

Write the addition equation:

$$12+\square=27$$

The unknown is a part, so use subtraction to find it. Subtraction is the inverse, or opposite, of addition:

$$27-12=15$$

So, $\boxed{15}$ storybooks are in the library.

Check your answer with addition:

$$12+15=27$$

The parts, 12 and 15, make the whole, 27.