Chapter 1.7, Problem 37P

Use mental math to find each product.

$4.2\times 5$

To determine

To find the product:

  $4.2\times 5$

Answer to Problem 37P

21

Explanation of Solution

Given:

  $4.2\times 5$

Concept Used:

To multiply the product of decimals follow the steps:

1. First multiply the numbers ignoring the decimal.
2. Count the number of digits after the decimal in each factor.
3. Place the decimal point in the answer of first step behind the number of digits combined in each factor behind the decimal.

Calculation:

In order to find the product:

  $4.2\times 5$

Fist simply multiplying the above number ignoring the decimal, as

  $42\times 5=210$

Now, note down the number of digits after decimal point in each factor of given product.

Since, 4.2 have one digit after decimal point and 5 have no points after decimal.

So the final answer will have only 1 digit after decimal point.

Thus,

  $\begin{array}{c}4.2\times 5=21.0\\ =21\end{array}$