30% of the people are performers for a concert and the rest are spectators. 80% of the spectators are parents and there are 2100 more parent spectators than child spectators. How many parent spectators must leave so that 12% of the people are parent spectators?

	Performers	Spectators
	3x5	7x5
		Children	Parents
		1x7	4x7
Before	15 u	7 u	28 u
Change			- ?
After	22x1 = 22 u		3x1 =  3 u

30% = ³⁰₁₀₀ = ³₁₀ 

80% = ⁸⁰₁₀₀ = ⁴₅ 

12% = ¹²₁₀₀ = ³₂₅

The total number of spectators at first is repeated.  Make the total number of spectators the same.  LCM of 7 and 5 is 35.

Number of more parent spectators than child spectators at first
= 28 u - 7 u
= 21 u

21 u = 2100

1 u = 2100 ÷ 21 = 100

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

Number of parent spectators who must leave so that 12% of the people are parent spectators
= 28 u - 3 u
= 25 u
= 25 x 100
= 2500

Answer(s): 2500