At first, flour to sugar is 4:1
To bake each cake is 3:1
3 units - 1unit = 1200g
2 units =1200g
1 unit = 1200 / 2 = 600g (sugar)
4 units = 2400g (flour)
600g + 1200g = 1800g (flour used)
2400g - 1800g = 600g (flour left)

Hope my explanation is clear enough