In a school, 0.8 of the girls and 0.1 of the boys take part in competitions. There is an equal number of boys and girls who do not take part in competitions. If 666 students take part in competitions, how many less students do not take part in competitions than those who take part in competitions?

Girls		Boys
Take part	Do not take part	Take part	Do not take part
4x9	1x9	1	9
	1x9		1x9
36 u	9 u	1 u	9 u

0.8 = ⁴₅
Girls that take part : Girls that do not take part = 4 : 1

0.1 = ¹₁₀
Boys that take part : Boys that do not take part = 1 : 9

The number of boys who do not take part is repeated.  Make the number of boys who do not take part the same.  LCM of 1 and 9 is 9.

Number of students who take part in competitions
= 36 u + 1 u
= 37 u

37 u = 666

1 u = 666 ÷ 37 = 18

Number of students who do not take part in competitions
= 9 u + 9 u
= 18 u

Number of less students who do not take part in competitions than those who take part in competitions
= 37 u - 18 u 
= 19 u
= 19 x 18
= 342

Answer(s): 342