Mental Maths Class 5 Knowing Our Numbers

Asked by AKSHAY GURU VARADHARAJAN · SRI CHAITANYA TECHNO SCHOOL, JAYANTI NAGAR.

How many numbers ≤ 1000 have exactly 3 factors?

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A 10
B 11
C 12
D 13

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Answer: B. 11

Numbers with exactly 3 factors are of the form p^2

So:
p^2 <= 1000 => p <= sqrt(1000) ≈ 31

Prime numbers <= 31:
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31

Total = 11

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How many numbers ≤ 1000 have exactly 3 factors?

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