Question

Ludka has written out the first 100 numbers and has put colored boxes around the numbers she thinks are prime.

She eliminated all numbers over 5 that end with a 0, 2, 4, 5, 6 and 8 and all multiples of 3.

But there are three numbers that are not prime that she has colored in by mistake.

What are they and why are they not prime numbers?

A chart with natural numbers 1 through 100. The numbers 11, 31, 41, 61, 71, 91 are blue. The numbers 2 and 5 are orange. The numbers 3, 13, 23, 43, 53, 73, 83 are green. The numbers 7, 17, 37, 47, 67, 77, 97 are red. The numbers 19, 29, 49, 59, 79, 89 are purple.
Select answers

49 because 7 is a factor.
59 because 13 is a factor.
71 because 11 is a factor.
77 because 11 is a factor.
91 because 7 is a factor.
97 because 13 is a factor.

Answer 1

The three composites she missed are 49, 77 and 91.

- 49 = 7 × 7
- 77 = 7 × 11
- 91 = 7 × 13

She only eliminated multiples of 2, 3 and 5 (and numbers ending 0,2,4,5,6,8), so composites with other small prime factors (like 7, 11, 13) stayed colored.