25% of the people are performers for a concert and the rest are spectators. 60% of the spectators are adults and there are 210 less child spectators than adult spectators. How many adult spectators must join so that 89% of the people are adult spectators?

	Performers	Spectators
	1x5	3x5
		Children	Adults
		2x3	3x3
Before	5 u	6 u	9 u
Change			+ ?
After	11x1 = 11 u		89x1 =  89 u

25% = ²⁵₁₀₀ = ¹₄ 

60% = ⁶⁰₁₀₀ = ³₅ 

89% = ⁸⁹₁₀₀ = ⁸⁹₁₀₀

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 = 210

1 u = 210 ÷ 3 = 70

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

Number of adult spectators who must join so that 89% of the people are adult spectators
= 89 u - 9 u
= 80 u
= 80 x 70
= 5600

Answer(s): 5600