There are 3 separate pieces of timber - they cannot be joined together to make planks. We need to make many planks of equal length. We do not want to waste any timber.
It we cut 42 m long timber from all the three pieces, then there is no waste from the 1st one, there is a waste of 7m from 2nd timber and 21 m length is wasted from the third . We get 3 planks of 42 m length.
If we take HCF of all the given lengths (42, 49, 63) , it is 7 m. Hence we make planks of length 7m, then we get 6+7+9 = 22 planks of equal length 7 m.