81 divided by 37.5 equals 2.16, 2.16 would be equivalent to 1% so to find 100% we simbly multiply 2.16 by 100, 2.16 multiplied by 100 equates to 216, so the Baker is making 216 muffins.
If this is a word problem, put it into your own words in case of getting in trouble.
The equation for percentage increase is {(New value - old value) / old value} x100% Therefore it should be {(22.5 - 19.5) / 19.5} x 100% =15% (rounded to nearest whole number) Answer is B