Let x be the number of $1 bags that we bought.
An, let y be the number of $2 bags that we bought.
So the total cost equation is:
We may re-write this as:
`x + 2y = 600`
Since the number of $1 bags (x) is twice the number of $2 bags (y), then:
`x = 2y`
Substitute this to the total cost equation.
Divide both sides by 4, to isolate the y.
So, we bought 150 pieces of $2 bag.
Then, substitute this to x = 2y.
And, we ordered 300 pieces of $1 bag.
Then, add the values of x and y to get the total number of bags.
`x + y = 300+150 = 450`
Hence, we bought a total of 450 bags.