Find the least common multiple of $24,42$ and $60$.

Let's write Prime factors of given No.s

$24=2\times 2\times 2\times 3$

$42=2\times 3\times 7$

$60=2\times 2\times 3\times 5$

Hence, L.C.M. $=2\times 2\times 2\times 3\times 7\times 5=840$