Answer:
55g NH₄Cl / 100g Water
Explanation:
Solubility of a substance define the amount of solute per solvent in a saturated solution. The solution can dissolve additional solute if heated.
In the problem, as the first crystal appears at 61°C the solubility in this temperature is the concentration of the solution, that is:
2,75g NH₄Cl / 5,0g water ₓ 100 = 55g NH₄Cl / 100g Water
I hope it helps!