Answer:

The lengths of three pieces of timber are 42m, 49m and 63m

Divide the timber into equal length of planks.

∴ Greatest possible length of each plank = HCF (42, 49, 63)

Using prime factorization,

42 = 2 × 3 × 7

49 = 7 × 7

63 = 3 × 3 × 7

∴HCF = 7

The greatest possible length of each plank = 7m.