No, that's a 6 percentage point increase, which is a 40% increase: 21 is 140% of 15!
It’s not a matter of the math being right because we are dealing with people who have been taught Kommie Kore math. What matters is showing the increase. We both know it is a far greater rate than 6%. Showing that it is an increase in any form will show that indeed it is an increase.