A man can do a piece of work in 5 days, but with the help

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A man can do a piece of work in 5 days, but with the help of his son, he can do it in 3 days. In what time can the son do it alone ?

Using Rule 2,
Let the son take x days to do the work.

∴	1	+	1	=	1
	5		x		3

⇒	x + 5	=	1
	5x		3

⇒ 3x + 15 = 5x

⇒ 2x = 15 ⇒ x=	15
	2

7	1	days
	2