Build the percentage from 10% (÷10) and 5% (half of 10%): e.g. 15% = 10% + 5%. The correct answer is 21.0. Traps to avoid: 35.0 comes from the "wrong benchmark" error; 2.10 comes from the "decimal slip" error; 28.0 comes from the "wrong benchmark" error; 14.0 comes from the "used 10pct" error.