How do you do the math problem 7.8 times 0.03?

Answer 1

When doing it by pencil and paper (or mentally):

1) ignoring the decimal points multiply the numbers together;

2) count how many digits in total are after the decimal points in the multiplication;

3) put the decimal point into the answer (inserting leading zeros if necessary) so that the same number of digits follow the decimal point;

4) any trailing zeros can be not written.

For 7.8 × 0.03 this leads to:

78 × 3 = 234

in 7.8 there is 1 digit after the decimal point

in 0.03 there are 2 digits after the decimal point

Thus in 7.8 × 0.03 there are 1 + 2 = 3 digits in total after the decimal point, and the answer must also have 3 digits after the decimal point.

→ 7.8 × 0.03 = 0.234

If the question had been 0.05 × 0.4 this leads to:

5 × 4 = 20

0.05 × 0.4 has 2 + 1 = 3 digits after the decimal point

Thus 0.05 × 0.4 = 0.020 which has 3 digits after the decimal point but would [normally] be written without the trailing zero as 0.02 (only having 2 digits after the decimal point).