You know that a #"2% m/m"# sucrose solution must contain #"2 g"# of sucrose for every #"100 g"#, so right from the start you can say that the total mass of the solution will be equal to #"100 g"#.
Now, you're making this solution by adding #"2 g"# of sucrose to water, so use the total mass of the solution to figure out exactly how much water you need
#"2 g sucrose"color(white)(.) + xcolor(white)(.)"g water" = "100 g solution"#
You will end up with
#x = "100 g" - "2 g"#
#x = "98 g"#
You got to the answer by subtracting your value from a constant, i.e. from #"100 g"# of solution, so the answer must not have any decimal places because of the fact that #"2 g"# of sucrose doesn't have any decimal places.
Therefore, you can say that in order to make a #"2% m/m"# sucrose solution, you need to add #"2 g"# of sucrose to #"98 g"# of water.
