Basic algebra help please?

Well using what u know u can say that the number of crates of melons plus apples equals 48. You can also say that 2o dollar melons plus 8 dollar apples equals the total cost of 420. So u know two equations m+a=48 and 20m+ 8a=420 solve for one and then plug it into the other equation to get a value for how many crates of apples or melons were sold.... Hope that helps