40% of the people are performers for a concert and the rest are spectators. 60% of the spectators are adults and there are 210 more adult spectators than child spectators. How many adult spectators must leave so that 20% of the people are adult spectators?

	Performers	Spectators
	2x5	3x5
		Children	Adults
		2x3	3x3
Before	10 u	6 u	9 u
Change			- ?
After	4x4 = 16 u		1x4 =  4 u

40% = ⁴⁰₁₀₀ = ²₅ 

60% = ⁶⁰₁₀₀ = ³₅ 

20% = ²⁰₁₀₀ = ¹₅

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 more adult spectators than child spectators at first
= 9 u - 6 u
= 3 u

3 u = 210

1 u = 210 ÷ 3 = 70

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

Number of adult spectators who must leave so that 20% of the people are adult spectators
= 9 u - 4 u
= 5 u
= 5 x 70
= 350

Answer(s): 350