Answer :
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
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