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\\October 8, 2025 12:13 PM Bharat Arora P-9 Reader _________________________ Date: ___________________74

Step 4: Next, the suitable prime number is 5; divide the numbers by 5.
Step 5: Continue dividing till you get 1 as the quotient for all the numbers.

Thus, the LCM of 15, 20, 25 and 45 = 3 × 5 × 2 × 2 × 3 × 5 = 900.
example 4: Find the LCM of: (a) 20, 60 and 90 (b) 24, 40 and 72

Solution: (a) 2 20, 60, 90 (b) 2 24, 40, 72
2 10, 30, 45 2 12, 20, 36
3 5, 15, 45 2 6, 10, 18
5 5, 5, 15 3 3, 5, 9
3
3 1, 5,
3 1, 1, 3 5 1, 5, 1
1, 1, 1 1, 1, 1

LCM = 2 × 2 × 3 × 3 × 5 = 180 LCM = 2 × 2 × 2 × 3 × 3 × 5 = 360

Practice time 3F

1. Write three common multiples of each pair of numbers. Circle their smallest
common multiple.
( a) 12, 20 (b) 16, 24 (c) 14, 21 (d) 18, 30 (e) 10, 15

2. Find the LCM of these numbers using the common multiples method.

( a) 21, 28 (b) 48, 72 (c) 22, 110 (d) 60, 63
( e) 24, 36 (f) 38, 57 (g) 12, 15, 18 (h) 20, 30, 50
( i) 20, 32, 40 (j) 11, 22, 33 (k) 12, 15, 40 (l) 12, 16, 32

3. Find the LCM of these numbers using the prime factorisation method.

( a) 14, 17 (b) 51, 54 (c) 78, 82 (d) 25, 75
( e) 54, 90 (f) 24, 72 (g) 18, 20, 32 (h) 12, 16, 30
( i) 21, 24, 36 (j) 3, 15, 60 (k) 33, 55, 99 (l) 12, 18, 24

4. Find the LCM of these numbers using the division method.
( a) 4, 24, 32 (b) 24, 42, 72 (c) 20, 60, 90 (d) 18, 36, 48

( e) 9, 13, 26 (f) 18, 9, 27 (g) 32, 16, 50 (h) 14, 35, 49

5. Two bulbs flash at regular intervals of 42 and 77 seconds respectively. They first flash
together at 10:45 p.m. At what time will they flash together for the:
( a) Second time? (b) Fifth time?

6. Sia has a collection of hair bands and she wants to arrange them in groups of 4, 6 or
8 with no hair band left behind. Find the number of hair bands she has.

7. Two bells ring at intervals of 24 min and 36 min respectively. If they ring together at
8:15 a.m., when will they ring together next?
8. What is the smallest number that, when divided by 20, 25, and 35, leaves a remainder

of 5 in each case?

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