That's soooo wrong. 2 Cubic Yards does not equal 18 cubic feet. There are 27 cubic feet in 1 Cubic Yard. 3ft.x 3ftx3ft = 27 cubic feet. 2 Cubic Yards would be 54 Cubic feet. If a 50 lb bag of gravel is roughly 0.6 cubic feet then you would need 90 bags...54/0.6 = 90

Weight doesn't exactly translate to volume, but generally I think a 50 lb bag is approximately 0.6 cubic feet. Bag labels may give you a volume measurement for the particular brand you are looking at using.

2 cubic yards = 18 cubic feet, so 18/0.6 = 30 bags