10 Brain Teasers for Class 9 with Answers
Brain teasers train you to slow down, read the question carefully, and check your assumptions. Most of the puzzles below need no formulas at all. Try each one before you read the answer, then check your reasoning against the explanation.
The puzzles
- Bat and ball. A bat and a ball cost ₹110 together. The bat costs ₹100 more than the ball. How much does the ball cost?
- Lily pad. A lily pad doubles in size every day. It covers the whole lake on day 48. On what day does it cover half the lake?
- Machines. Five machines take 5 minutes to make 5 widgets. How long do 100 machines take to make 100 widgets?
- Family. Mary's father has five daughters: Nana, Nene, Nini and Nono. What is the name of the fifth daughter?
- Apples. You have three apples and you take away two. How many apples do you have?
- Total. What is the sum of all whole numbers from 1 to 100?
- Half and add. What is half of two, plus two?
- Clock. A digital clock shows 11:11. How many minutes until it shows 12:00?
- Weight. Which weighs more: one kilogram of feathers or one kilogram of iron?
- Letters. What comes next in the sequence: O, T, T, F, F, S, S, ?
Answers and explanations
1. ₹5. Let the ball cost x. Then the bat costs x + 100, and x + (x + 100) = 110. So 2x = 10 and x = 5. The bat costs ₹105 and the total is ₹110. Many people instinctively answer ₹10, which gives a total of ₹120.
2. Day 47. If the pad doubles each day, it is half covered the day before it is fully covered. Half of the lake is covered on day 47.
3. 5 minutes. Each machine makes one widget in 5 minutes. So 100 machines, working at the same time, make 100 widgets in the same 5 minutes.
4. Mary. Mary's father has five daughters, and Mary is the fifth. The puzzle tests whether you read the whole question.
5. Two. You took two apples, so you have two. Read the final question, not the sum you might be tempted to calculate.
6. 5050. Pair the numbers from the ends: 1 + 100, 2 + 99, and so on. There are 50 pairs of 101 each, so 50 × 101 = 5050. Another route is the formula n(n + 1) / 2 with n = 100.
7. 3. Half of two is 1. Then 1 + 2 = 3.
8. 49 minutes. From 11:11 to 12:00 is 60 − 11 = 49 minutes.
9. They weigh the same. Each is one kilogram. The material does not change the weight.
10. E. The letters are the first letters of the numbers one, two, three, four, five, six, seven. The next number is eight, which begins with E.
How to get better at brain teasers
- Write down exactly what the question asks before you calculate anything.
- Check whether a simple answer fits every part of the wording.
- When a puzzle feels too easy, reread it. The trick is often in one word.
Keep practising
Try a full 10-question challenge for Class 9 on BrainSprint. Play a Class 9 challenge. You can also read our number-series guide for the next step.
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