Weight of the watermelon when bought = 5 kg 200 g = 5.200 kg (we know that 1 kg = 1000 g)
Weight of the watermelon given to the neighbour = 2 kg 750 g= 2.750 kg
Therefore, weight of the watermelon left with Ruby = Weight of the watermelon when bought - weight of the watermelon given to the neighbour
= 5.200 kg - 2.750 kg
= 2.450 kg
So, weight of the watermelon left with Ruby = 2.450 kg