10.4%
First, calculate the total mass by adding the masses together.
26.0 g + 225 g = 251 g
Now calculate the percentage of 251 that 26 is using division:
26 / 251 = 0.103585657 = 10.3585657%
So the mass percent of KCl is 10.4% (after rounding to 3 significant digits)