Question

A boy was asked to find the LCM of 3.5. 12 and another number. But
answer. What could be the fourth number?
while calculating, he wrote 21 instead of 12 and yet came with the correct​

Answer 1

Step-by-step explanation:

The prime factors of 3, 5, 12 are 3, 4, 5

and the prime factors for 3, 5, 21 are 3, 5, 7

In order for the LCM to be the same the prime factors have to be the same. but in the first case 7 is missing and in the second case 4 is missing. So, the answer should be a product of these two numbers ie 28.

Now, with 28, the prime factors for 3, 5, 12, 28 are 3, 4, 5, 7 and the LCM will be 3 * 4* 5 * 7 = 420

and as with 21 the prime factors for 3, 5, 21, 28 are 3, 4, 5, 7 and the LCM again is 3 *4 * 5* 7 = 420

Hence 28 is the answer.

Note: There are many answers to the question and 28 is the smallest possible number and all the multiples of 28 are possible solutions for this question.