Three piece of timber 42 m , 49 m  and 63 m long have to be divided into planks of the same length. what is greatest possible length of each plank ? How many planks are formed?

Given : the lengths of three pieces of timber are $42$ m$,49$ m and $63$ m respectively. we have to divide the timber into equal length of planks.

greatest possible length of each plank $=HCF(42,49,63)$

The prime factorization of $42,49$ and $63$ are:
$42=2\times 3\times 7$
$49=7\times 7$
$63=3\times 3\times 7$

HCF $(42,49,63)=7$

the greatest possible length of each plank is $7$ m.
again, the number of planks can be formed $=22.$