adding-and-subtracting-decimals MCQs for UPSC Prelims

25 practice questions on adding-and-subtracting-decimals from the A Peek Beyond the Point section of the UPSC Prelims syllabus. 25 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

7 Easy 13 Medium 5 Hard

Sample questions

Q1
hard

A student works out $12.6-8.9$ and writes $4.7$. What does estimation tell us about this answer?

  1. A It is right; the estimate is also $4.7$
  2. B It is wrong; the answer should be about $21$
  3. C It is right, because estimation cannot check a subtraction
  4. D It is wrong; the answer should be about $4$, and exactly $3.7$
Show answer and explanation

Correct answer: D - It is wrong; the answer should be about $4$, and exactly $3.7$

Rounding gives $13-9=4$, and the exact difference is $3.7$, so the written answer $4.7$ is a whole unit too large. An estimate near $21$ would come from adding the two numbers, and estimation is a perfectly good check on a subtraction.

Q2
medium

One jug holds $1.5$ litres of juice and another holds $0.85$ litre. How much juice is there altogether?

  1. A $1.00$ litre
  2. B $1.35$ litres
  3. C $2.35$ litres
  4. D $2.30$ litres
Show answer and explanation

Correct answer: C - $2.35$ litres

Writing $1.5$ as $1.50$ and adding gives $1.50+0.85=2.35$ litres. Reading $1.5$ as $0.15$ gives $1.00$ litre, forgetting the carry into the ones column gives $1.35$, and $2.30$ drops the five hundredths.

Q3
easy

A notebook costs ₹$25.50$ and a pen costs ₹$14.50$. What is the total cost?

  1. A ₹$11$
  2. B ₹$39$
  3. C ₹$40$
  4. D ₹$40.50$
Show answer and explanation

Correct answer: C - ₹$40$

The paise add to $50+50=100$, which is one whole rupee, so $25.50+14.50=40$. The amount ₹$39$ misses that carry, ₹$40.50$ carries the rupee and keeps the paise as well, and ₹$11$ subtracts instead of adding.

Q4
medium

What is $9+0.6$?

  1. A $15$
  2. B $0.96$
  3. C $9.06$
  4. D $9.6$
Show answer and explanation

Correct answer: D - $9.6$

The whole number $9$ can be written as $9.0$, and $9.0+0.6=9.6$. The value $0.96$ shrinks the $9$ into the tenths place, $15$ adds $9$ and $6$ as whole numbers, and $9.06$ puts the $6$ in the hundredths place.

Q5
easy

When adding $4.5$ and $12.35$ in columns, what must be placed one below the other?

  1. A The last digits on the right
  2. B Nothing in particular
  3. C The first digits on the left
  4. D The decimal points
Show answer and explanation

Correct answer: D - The decimal points

Adding decimals works only when digits of the same place value meet, and lining up the decimal points does exactly that. Lining up the rightmost or leftmost digits would put tenths under hundredths and give a wrong total.

Q6
medium

A pot holds $2.75$ litres of milk. If $1.4$ litres is used for tea, how much milk is left?

  1. A $2.61$ litres
  2. B $1.35$ litres
  3. C $1.45$ litres
  4. D $1.71$ litres
Show answer and explanation

Correct answer: B - $1.35$ litres

Writing $1.4$ as $1.40$ gives $2.75-1.40=1.35$ litres. Reading $1.4$ as $1.04$ gives $1.71$, reading it as $0.14$ gives $2.61$, and $1.45$ slips by one tenth in the tenths column.

Q7
medium

What is $10-4.35$?

  1. A $5.65$
  2. B $5.75$
  3. C $6.35$
  4. D $6.65$
Show answer and explanation

Correct answer: A - $5.65$

Writing $10$ as $10.00$ and borrowing across the zeros gives $10.00-4.35=5.65$. The value $6.65$ subtracts from $11$, $5.75$ comes from $10.10-4.35$, and $6.35$ subtracts only the whole number $4$.

Q8
medium

Without working it out exactly, which is the best estimate of $19.8-4.95$?

  1. A About $15$
  2. B About $25$
  3. C About $150$
  4. D About $5$
Show answer and explanation

Correct answer: A - About $15$

Rounding to whole numbers gives $20-5=15$, close to the exact value $14.85$. The estimate $25$ comes from adding instead of subtracting, $5$ is far too small, and $150$ shifts the decimal point.

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