Word Problem of System of Equation

Q. Jimmy is 12 years older than Brandon. 17 years ago, Jimmy was 4 times as old as Brandon. How old is Brandon now?

Suppose Jimmy's current age is J and Brandon's age is B. Jimmy is 12 years older than Brandon then
J = B + 12
17 years ago Jimmy was 4 times as old as Brandon.
So, 17 years ago Jimmy's age was J-12 and Brandon's age was B-17 then
J-17 = 4(B - 17)
Put J = B + 12 in above equation.
B + 12 - 17 = 4( B - 17 )
B - 5 = 4B - 68
3B = 63
B = 21
Brandon is 21 years old.

Q. Your class has 40 students and some want to watch movie and some want to watch stage drama. The cost of movie ticket is 20$ and stage drama is 10$. Total cost of ticket is 500$. How many students went to watch movie and how many want to watch stage drama.

M= Number of student to watch movie
S = Number of student to watch stage drama

We know that total student is 40. Then
M+S = 40 
We also know total cost of ticket is 500$. Then
20M + 10S = 500
Put M = 40 - S  in above equation, it will become
20(40 - S) + 10S = 500
800 - 20S + 10S= 500
800 - 10S = 500
10S= 300
S= 30

Put this value in M+S = 40 then M + 30 = 40.
M = 10
30 students went to watch stage drama and 10 students went to watch movie.