How you know the number of decimal places that should be in the product when you multiply two decimal numbers together?

Answer 1

Count the number of digits after the decimal point in the first number being multiplied.

Count the number of digits after the decimal point in the second number being multiplied.

Add the two numbers.

This is the number of digits after the decimal point in the answer. But make sure that any trailing 0s are included.

To calulate

0.42 * 3.451:

41*3451 = 144942

Digits after point in first number = 2

Digits after point in second number = 3

2 + 3 = 5

So digits after point in answer = 5

So answer = 1.44942

But

To calulate

0.42 * 3.45:

42*345 = 14490 <<< it is vital that the last 0 here is kept till the last stage.

Digits after point in first number = 2

Digits after point in second number = 2

2 + 2 = 4

So digits after point in answer = 4

So answer = 1.4490 = 1.449