10a+ 10c + s = 180
5a + 3c + 2s = 500
a = 10c
replace a with the 10c in the first two equations.
Combine the the c terms.'
Now, you have two equations in two variables and you can solve by using the elimination/addition method where you add to eliminate one of the variables.
Are you sure you have 10a or was it just a in the first equation?