If 45.8 grams of mgo are dissolved in 205 grams of water, what is the concentration of the solution in percent by mass

Although it's true that MgO is only very slightly soluble in water, you can consider the mixture a conventional solution for the purposes of this question. The % mass is the mass of MgO divided by the mass of the entire solution:

45.8 / (45.8 + 205) * 100 = 18.3%

Abbreviated as mg-25.