20% of the people are performers for a concert and the rest are spectators. 70% of the spectators are parents and there are 1280 less child spectators than parent spectators. How many parent spectators must join so that 89% of the people are parent spectators?

	Performers	Spectators
	1x5	4x5
		Children	Parents
		3x2	7x2
Before	5 u	6 u	14 u
Change			+ ?
After	11x1 = 11 u		89x1 =  89 u

20% = ²⁰₁₀₀ = ¹₅ 

70% = ⁷⁰₁₀₀ = ⁷₁₀ 

89% = ⁸⁹₁₀₀ = ⁸⁹₁₀₀

The total number of spectators at first is repeated.  Make the total number of spectators the same.  LCM of 4 and 10 is 20.

Number of less child spectators than parent spectators at first
= 14 u - 6 u
= 8 u

8 u = 1280

1 u = 1280 ÷ 8 = 160

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

Number of parent spectators who must join so that 89% of the people are parent spectators
= 89 u - 14 u
= 75 u
= 75 x 160
= 12000

Answer(s): 12000