Multiplication with Money

When students are first asked to multiply with money they typically multiply an amount that contains dollars and cents with a whole number.  This is a great introduction to multiplication with decimals.  The main multiplication with decimal rules apply:

• Students do not have to line numbers up by the decimal. Lots of students like to do this anyway, but this introduces a lot of unnecessary decimals and space for careless errors.  Try to explain that multiplication is different and does NOT require lining up decimals. 
• Students count the number of digits behind decimals in the factors to determine how many digits should be behind the decimal in the answer.

But, because we are dealing with money (and only one amount with a decimal), there are always two digits behind the decimal point.

Example: 

$5.23 \times 7 = $

Write the problem vertically.

$$\eqalign{5.23\\\times 7\\\hline\qquad}$$

Multiply. Place the decimal two digits in in the answer.

$$\eqalign{\overset{1}{5}.\overset{2}{2}3\\\times 7\\\hline 36.61}$$

The process of multiplying with money does give students a start working with decimals (and, importantly, not forgetting to include the decimal in their answers), but be careful not to let students think that multiplying with decimals is always so automatic.