The first step we have to follow is to add the given mass to find the total mass of the solution:
[tex]M=15.4g+74.6g=90g[/tex]Now, divide the mass of the solute by the mass of the solution and multiply it times 100 to find the percent by mass:
[tex]\%m/m=\frac{15.4g}{90g}\cdot100=17.1\%[/tex]The percent by mass of the solute in the solution is 17.1%.