Givens
Red Cards = R = 5 point.
Black Cards =B = -10
Equations
R = 7B
5*R - 10*B = 75
Substitute for R
5*7B - 10B = 75
35B - 10B = 75 Subtract 10 from 35
25B = 75 Divide by 25
B = 75/25
B = 3
Find R
R = 7B
R = 7*3
R = 21
Check
5R - 10B = 75
5*21 - 10*3 = ? 75
105 - 30 =? 75
75 = 75 And it checks.
Answer
R = 21
B = 3
