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Example 3: Find the prime factorisation of 80 by using the division method.
Solution: To find the prime factorisation of 80, we follow the given steps:

Step 1: Divide 80 by the smallest prime number 2 mentally, and 2 80
write the quotient 40 below 80. 2 40
Step 2: Again, divide 40 by 2. Write the quotient 20 below 40. 2 20

Step 3: Continue the process, till we get the quotient as a prime 2 10
number. Here, it is 5. 5
Step 4: Express 80 as a product of all the divisors and the last quotient.

Thus, the prime factorisation of 80 = 2 × 2 × 2 × 2 × 5.

Practice time 5D

1. identify whether the following numbers are prime or composite.
(a) 15 (b) 21 (c) 23 (d) 33 (e) 46 (f) 49
2. Write all the prime numbers between:

(a) 10 and 30 (b) 50 and 60 (c) 80 and 90
3. how many composite numbers are there between two given numbers?

(a) 1 and 10 (b) 20 and 30 (c) 70 and 80
4. Complete the factor tree by filling the missing factors.

(a) 30 (b) 63 (c) 100

× 3 9 × ×

×
2 × ×

×
5. Fill in the boxes to complete the prime factorisation by the division method.

(a) (b) (c)
75 2 625

25 2 54

9 5

3
6. Find the prime factorisation of the following numbers.

(a) 18 (b) 20 (c) 32
(d) 42 (e) 60 (f) 81

Mathematics-4 101