Each mixture has one part lawn feed and two parts water. That means each mixture has 50ml lawn feed and 2 x 50ml = 100ml water.
So, each mixture has 100ml of water and we know it will cover 1.5m2 of garden.
That means for 4.5m2 = 3 x 1.5m2 we will need 3 x 100ml = 300ml of water.