At 60C how much potassium nitrate can be dissolved in 400g of water

About   110 g

Explanation:

The solubility graph is a very useful tool because it tells you the amount of potassium nitrate that can be dissolved per  

100 mL

of water in order to have a saturated solution of potassium nitrate at a given temperature.

In order to find the solubility of the salt at  60  degree celcius

start from the  60  degree celcius

Mark on the graph and move up until you intersect the curve. At the point of intersection, move left until you intersect the  y axis and read off the value that you land on.

In this case, you can estimate that the solubility of potassium nitrate in water at  60 degree celcius is equal to about

solubility  ≈ 110 g / 100 mL water

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−  

This tells you that a saturated solution of potassium nitrate will hold about  

110 g  of dissolved salt, i.e. of dissociated salt, per  100 ml of water at 60 degree celcius

that tool would be a compass