You have answered your own question: 4e+2c≤20 says the total cost of egg cake is 4e dollars and the total cost of cream cake is 2c dollars, and the sum must not exceed $20, must be less than or equal to $20.
The equation can be divided by 2: 2e+c≤10 or c≤10-2e. This shows clearly the relation between c and e. If, for example, e=3 then c≤4.