Given in the question that,

Weight of rice = 5 kg 400 g

= (5 + ) kg

= 5 + 0.400 kg

= 5.400 kg

Weight of Sugar = 2 kg 20 g

= (2 + ) kg

= 2 + 0.020 kg

= 2.020 kg

Also,

Weight of flour = 10 kg 850 g

= (10 + ) kg

= 10 + 0.850

= 10.850 kg

Therefore,

Total weight = 5.400 + 2.020 + 10.850

Hence, total weight of purchases = 18.270 kg