Question

A boy was asked to find the LCM of 3, 5, 12 and another number. But
while calculating, he wrote 21 instead of 12 and yet came with the correct
answer. What could be the fourth number?

Answer 1

any multiple of 28

LCM(3, 5, 12) = 60

LCM(3, 5, 21) = 105

LCM(60, 105) = 420

LCM(3, 5, 12, x) = LCM(3, 5, 21, x) = 420*n = 60*7*n = 105*4*n

x = 4*7*n where n = {1,2,3,...}