Adding Tenths and Hundredths

3/10 + 25/100 has two different units, so the pieces cannot be counted yet.

Rewrite the tenths

3/10 is 30/100. Both are hundredths at that point.

Then count

30 hundredths and 25 hundredths make 55 hundredths, which is 0.55.

The same rule as before

This is the fraction rule again: match the units first, then count them.

Convert to the same unit, then count

0.3 + 0.24 cannot be added as 3 and 24 — those are different units. Rewrite 0.3 as 0.30, giving 30 hundredths plus 24 hundredths = 54 hundredths, or 0.54. Same-unit-first is the rule for fractions and for decimals alike.

Line up the decimal points

Writing the numbers with their decimal points in a column automatically puts tenths above tenths and hundredths above hundredths. That alignment is the whole reason the column method works, and misaligning is the only way to get it badly wrong.

Check it as fractions

0.3 + 0.24 is 3/10 + 24/100 = 30/100 + 24/100 = 54/100 = 0.54. Doing it both ways confirms the answer and, more importantly, confirms that decimals and fractions really are the same numbers in different notation.

Estimate first

0.3 is about a third and 0.24 about a quarter, so expect a bit over a half. The answer 0.54 fits. Estimating with decimals is harder than with whole numbers, so relating them to familiar fractions is often the quickest route to a sense of size.