Fun with Numbers
Class 6 Maths · Chapter 5: Prime Time · NCERT Ganita Prakash section 5.6
Learn
Twin primes are primes that differ by 2: 3 and 5, 11 and 13, 29 and 31. Nobody knows if twin primes go on forever. It is another unsolved puzzle!
Try this: every even number from 4 onwards seems to be the sum of two primes. 10 = 3 + 7, 20 = 7 + 13, 36 = 17 + 19. People have checked huge numbers, but nobody has proved it is always true.
10 = 3 + 7 · 20 = 7 + 13 · 36 = 17 + 19
Who am I? I am a 2-digit prime. My digits add up to 7, and my tens digit is 1 more than my ones digit. Clues like these narrow things down step by step. Try it before you look in the Think box!
Explore: pick two primes that add up to the target.
- Tap two primes that add up to 30.
- Fresh board: tap two primes that add up to 24.
Remember
- Twin primes differ by 2 (11 and 13).
- Every even number from 4 seems to be a sum of two primes, but nobody has proved it.
Quick check
Which pair is twin primes?
- 7 and 9
- 13 and 15
- 17 and 19
- 21 and 23
Answer
17 and 19. 17 and 19 are both prime and differ by 2. 9, 15 and 21 are not prime.
Think about it
Solve the riddle: I am a 2-digit prime, my digits add up to 7, and my tens digit is 1 more than my ones digit.
One answer
Tens + ones = 7 and tens = ones + 1, so ones + 1 + ones = 7, which gives ones = 3 and tens = 4. The number is 43, and 43 is prime.
Practice
Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.