Page 65 - ICSE Math 5

A perfect number is a number which is equal to the sum of its all factors except the number itself.
For example, the smallest perfect number is 6 which is the sum of its factors 1, 2 and 3. Some
other perfect numbers are 28 and 496.

28 = 1 + 2 + 4 + 7 + 14 496 = 1 + 2 + 4 + 8 + 16 + 31 + 62 + 124 + 248

Exercise 5.1

1. Fill in the blanks.

(a) ___________ is the only even prime number.
(b) 1 is neither a ___________ nor a ___________ number.

(c) ___________ is the smallest composite number.

(d) Co-prime numbers have only ___________ as their common factor.
2. Find any five factors of the following numbers.

(a) 525 (b) 848 (c) 448 (d) 768

3. Find the prime factors of the following numbers using factor tree method.

(a) 248 (b) 150 (c) 46 (d) 398

4. Do as directed.
(a) Write the multiples of 18 which lie between 50 and 120.

(b) Write the composite numbers between 60 and 70.
(c) Write the prime numbers between 20 and 40.

5. Check whether the first number is a factor of the second number or not. Write Yes or No.

(a) 12; 1,428 (b) 13; 5,365 (c) 18; 12,312

6. Which of the following pairs are co-prime?
(a) 16 and 21 (b) 18 and 35 (c) 24 and 39

(d) 46 and 108 (e) 55 and 57

7. Sort out the prime, composite and pairs of twin prime numbers from the table.

1 2 3 4 5 6 7 8 9 10

11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30

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