When you add or subtract measured numbers, the answer keeps as many decimal places as the number with the fewest decimal places. It does not depend on total significant figures. Memory hook: "Add and subtract = count the dots (decimal places), not the digits." Example: 436.32 + 227.2 + 0.301 = 663.821, but 227.2 has only 1 decimal place, so the answer is rounded to 663.8.
Adding 436.32 + 227.2 + 0.301 gives 663.821, but 227.2 has only 1 decimal place, so the answer is rounded to 663.8 g. For addition and subtraction you match decimal places, not total significant figures.
Your doubts, answered
Do I count significant figures or decimal places when adding?
For addition and subtraction you count decimal places, not significant figures. Find the number with the smallest count of digits after the decimal point, and round your final answer to that many decimal places. (The significant-figure count rule is only for multiplication and division.)
Why is 436.32 + 227.2 + 0.301 = 663.8 and not 663.821?
Arithmetic gives 663.821, but the least precise number 227.2 is known only to 1 decimal place. Since the answer cannot be more precise than the worst measurement, you round 663.821 to 1 decimal place, giving 663.8 g.
What is the rule for subtraction like 9.99 m - 0.0099 m?
Subtract first: 9.99 - 0.0099 = 9.9801. Then look at decimal places: 9.99 has 2 decimal places (the least), so round the answer to 2 decimal places, giving 9.98 m. This is the exact NEET 2020 question.
Why does subtraction sometimes lose so many significant figures?
When two close numbers are subtracted, like 0.307 m - 0.304 m = 0.003 m, the result has only 1 significant figure even though the inputs had 3. This is called loss of significance. The decimal-place rule (3 decimal places here) is still what you apply.
Do I round before or after adding?
Add or subtract with all the digits first, then round the final answer once, at the very end. Rounding each number before adding builds up extra error. Do the full calculation, then apply the decimal-place rule to the result.
⚠️ The NEET trap ✗ Using the multiplication rule and keeping the least number of significant figures, e.g. writing 436.32 + 227.2 + 0.301 as 664 g. ✓ Use the decimal-place rule for addition and subtraction: 227.2 has 1 decimal place, so the answer is 663.8 g (1 decimal place), not 664 g. 🧠 NCERT warns you directly: do NOT apply the multiply/divide significant-figure rule to add/subtract. Add and subtract are about decimal places only.
Real NEET questions
NEET 2020
Taking into account the significant figures, what is the value of 9.99 m - 0.0099 m?
A · 9.980 m
B · 9.9 m
C · 9.901 m
D · 9.98 m ✓
Solution: Step 1: Subtract exactly. 9.99 - 0.0099 = 9.9801 m. Step 2: Apply the subtraction rule. Count decimal places: 9.99 has 2 decimal places, 0.0099 has 4 decimal places. The least is 2. Step 3: Round 9.9801 to 2 decimal places. The third decimal digit is 0, so it rounds down, giving 9.98 m. Answer: D.
Solved Units And Measurements NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the rule for significant figures in addition and subtraction?
The final answer keeps as many decimal places as the number that has the fewest decimal places. You compare decimal places, not the total count of significant figures.
Is the addition rule different from the multiplication rule?
Yes. Addition and subtraction use decimal places. Multiplication and division use significant figures (the answer keeps the least number of significant figures). Mixing these up is the most common NEET mistake.
Does 227.2 g control the answer of 436.32 + 227.2 + 0.301?
Yes. 227.2 has only 1 decimal place, the smallest of the three, so the sum 663.821 g is rounded to 663.8 g, one decimal place.
Why does this matter for NEET?
NEET regularly asks one direct significant-figure question from Units and Measurements. Knowing the decimal-place rule lets you score it in seconds and avoid the trap answer.
When do I round the result?
Do the whole addition or subtraction first with all digits, then round the final answer once to the correct number of decimal places.
