Use the equations c = 100 - d and c = 20 + d to model the equation. Solve the system using substitution

You can substitute 20 + d for c in the first equation which gives you

20 + d = 100 - d then solve

2d = 80

d = 40

then put the value for d back into one of the original equations

c = 20 + d

c = 20 + (40)

c = 60