How many more grams of KI will dissolve in 100 g of water at 40 °C than at 20°C?

Your tool of choice here will be the solubility graph for potassium iodide, #"KI"#, which looks like this

Now, this solubility graph expresses the solubility of potassium iodide per #"100 cm"^3# of water, but you can use it as a measure of the salt's solubility per #"100 g"# of water.

Notice that at #40^@"C"#, a saturated potassium iodide solution will contain about #"195 g"# of dissolved potassium iodide.

At #20^@"C"#, on the other hand, a saturated potassium iodide solution will only contain about #"155 g"# of dissovled potassium iodide.

These values can be found by tracing vertical lines starting from #40^@"C"# and #20^@"C"#, respectively, until they intersect the solubility curve for potassium iodide. The values that correspond to these points represent the solubility of the salt in a saturated solution.

Therefore, you can say that a saturated solution of potassium iodide will hold #"40 g"# more potassium iodide at #40^@"C"# than at #20^@"C"#.

That is the case because

#overbrace("195 g")^(color(blue)("solubility at 40"^@"C")) - overbrace("155 g")^(color(purple)("solubility at 20"^@"C")) = "40 g"#