40% of the people are performers for a concert and the rest are spectators. 60% of the spectators are adults and there are 390 less child spectators than adult spectators. How many adult spectators must join so that 68% of the people are adult spectators?

	Performers	Spectators
	2x5	3x5
		Children	Adults
		2x3	3x3
Before	10 u	6 u	9 u
Change			+ ?
After	8x2 = 16 u		17x2 =  34 u

40% = ⁴⁰₁₀₀ = ²₅ 

60% = ⁶⁰₁₀₀ = ³₅ 

68% = ⁶⁸₁₀₀ = ¹⁷₂₅

The total number of spectators at first is repeated.  Make the total number of spectators the same.  LCM of 3 and 5 is 15.

Number of less child spectators than adult spectators at first
= 9 u - 6 u
= 3 u

3 u = 390

1 u = 390 ÷ 3 = 130

If some adult spectators join, the total number of child spectators and performers remains unchanged.  

Number of adult spectators who must join so that 68% of the people are adult spectators
= 34 u - 9 u
= 25 u
= 25 x 130
= 3250

Answer(s): 3250