F.1數學(英文)

Last month,there were 30 male students and 20 female students inthe chess class. The number of male students decreases by r% and thenumber of female students increases by r% in this month. If the numberof male and female students are the same in this month, find r.30x(1-r%) = 20x(1+r%)
30-30xr%=20+20xr%
10 = 50 x r%
1/5 = r%
r=20%

Your answer should be 20%!
30x(1-r%) = 20x(1+r%)
30-30xr%=20+20xr%
10 = 50 x r%
1/5 = r%
r=20