You have 40g of sodium chloride. What is the minimum amount of water required to dissolve the salt at 30oC?
The minimum mass of water is 111 g.
Step 1. Determine the solubility of
I am guessing that "they" are asking you to get the information from a solubility curve for sodium chloride like the one below.
(Adapted from http://www.sciencegeek.net/Chemistry/taters/solubility)
From 30 °C on the horizontal axis, draw a vertical line up until it hits the solubility curve (blue) for
Then draw a horizontal line to the left until it meets the vertical axis, and read the solubility.
The solubility is
Step 2. Calculate the mass of water required