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= 2×3×749=7×763=3×3×7HCF (42,49,63)=7the greatest possible length of each plank is 7 m.again, the number of planks can be formed =22.