The are 90 girls and 60 boys in the sixth grade at a middle school. Of these students, 9 girls and 3 boys write left-handed. What percentage of the sixth graders at this middle school write left-handed.

10 percent of the girls are left-handed and 5 percent of the boys are left-handed.

First, we write a fraction that expresses the number of sixth grades who can write left-handed out of all sixth graders. 9+3=12, 90+60=150, so the fraction is 12/150. 12 and 150 are both divisible by 6, since 1+2=3 and 1+5+0=6 and 12 and 150 are even, so we can divide 12/150 by 6/6, a form of 1, to get 2/25. Percentages are basically the numerator of a fraction with 100 as the denominator. Since 25 is a divisor of 100, we can simply multiply 2/25 by 4/4 to get 8/100. 8/100 as a percentage is 8%, which is our final answer.