A is 30% more efficient than B. How much time will they, working together, take to complete a job which A alone could have done in 23 days?

Ratio of times taken by A and B = 100 : 130 = 10 : 13.

Suppose B takes x days to do the work.

Then, 10 : 13 :: 23 : x          x =		23 x 13		x =	299	.
		10			10

(A + B)'s 1 day's work =		1	+	10		=	23	=	1	.
		23		299			299		13

Therefore, A and B together can complete the work in 13 days.