If a kilogram of wheat flower is $6. What is the cost of 400 grams of flour

1 kg = 1000 g

You can set it up as a proportion:

(1000g below is the measure of 1 kg.)

1000g/$6 = 400g/x

Find x.

Cross multiply:

x = (400*6) / 1000 = 2400/1000

x=2.4

It costs $2.40 to buy 400g of flour.