Question 814645
set c = number of cans
    b = number of bags
c + b = 120    
c = b + 30    we need to add 30 to number of bags to make these equal

If we substitute ( b+30 ) for c in
c + b = 120   we have
(b+30) + b = 120
2b + 30 = 120
Subtract 30 from each side
  2b = 120 - 30
  2b = 90
   b = 45
Since c = b+20
   c = 45+20
   c = 65