Potassium nitrate was found to have a solubility of 120 g in 100 g of water at 80 °C and 50 g in 100 g of water at 25 °C.

50 g of water was heated to 80 °C and solid potassium nitrate added until the solution was just saturated.

The solution was then cooled to 25 °C when solid potassium nitrate separated out to leave a saturated solution.

Using the information provided in this question, what is the minimum mass of water that must now be added to the mixture of the solution and the solid in order to make this solid potassium nitrate redissolve at 25 °C?

I’ll type in the solution since you probably cannot access the video explanation :))
so 50g of water at 80 C can dissolve 60 g of potassium nitrate (half of 120g) to be saturated. Then when it is cooled, some of the potassium nitrate solid would deposit as only 25g of potassium nitrate can dissolve in 50g of water at 25 C (half of 50 g)
So 60-25 = 35 g are deposited at the bottom. In order to dissolve 35 g of the solution, we can solve how much water we need to dissolve that at 25 C using ratios.
50/100=35/x
3500/50 = 70 g of water. Therefore the answer is D.