Good Morning. okay? To solve this problem is very simple, it is only using a system.
20c + 10b = 800
10c + 30b = 900
Multiply the second equation by -2.
20c + 10b = 800
-20c - 60b = -1800
Now it is only to add the two equations.
0 - 50b = -1000
b = -1000/-50
b = 20
Now that we have discovered the 'b' we will substitue in the equation.
10c + 30 . 20 = 900
10c + 600 = 900
10c = 300
c = 300/10
c = 30
b=20 c=30
ok? thanks, i hope i have helped.