21) 50 g

Maria takes a drink which has a volume of 500 ml and 4.0 Joules

100 g of the powder has 2.0 Joules

So Maria’s drink out have 200 g of the powder (2 x 2 = 4 J)

Nurse has 2 drinks (let us say A and B)

A has 1.5 Joules = 3/4 of 2 Joules

3/4 of 100 g = 75 g

A has 75 g of the powder.

B has 2 Joules in 400 ml

Since B has 2 Joules, it must have 100 g of the powder.

Concentration = 100 g/400 ml = 1 g per 4 ml

Since the Nurse pours 300 ml of B into A, mass of powder transferred = (300/4) x 1 = 75 g

Total mass of powder in new drink = 75 + 75 = 150 g

Since Maria needs 200 g, the Nurse must add = 200 – 150 = 50 g