If girls wearing eyeglasses are 20%, while girls with red hair are 15% you need to convert
them into fractions or decimals first, or you can also use percentage if you knew it, but I
highly recommend the usage of decimals to avoid errors.
This is the solution:
20% = 0.2 and 15% = 0.15
0.2×0.15 = 0.03 or move 0.03 to 2 decimal places to the left to become percent = 3% chance
of choosing a girl with eyeglasses and red hair.