Skill: Multiply decimals by whole numbers

Multiplying a decimal by a whole number means finding a whole-number number of equal groups.

For example, solve:

$$2.4 \times 3$$

This means 3 groups of 2.4:

$$2.4+2.4+2.4$$

Step 1: Estimate

Since $2.4$ is close to $2$, estimate:

$$2 \times 3=6$$

So the answer should be close to $6$.

Step 2: Decompose by place value

Break $2.4$ into $2$ ones and $4$ tenths:

$$2.4=2+0.4$$

Multiply each part by $3$:

$$3(2+0.4)=3\cdot2+3\cdot0.4$$ $$=6+1.2$$

Step 3: Add the products

$$6+1.2=7.2$$

Therefore,

$$\boxed{2.4\times3=7.2}$$

The answer makes sense because $7.2$ is close to the estimate of $6$, and $3$ groups of $2.4$ should be greater than $6$.