Let's focus on each grade individually for now. In grade 5, there are 90 students, with 10% absent. So, let's find 10% of 90. 90 is 100% in this case, so we'll need to divide 90 by 10, since 100/10 = 10 (we need things to stay proportional). So 9 students in 5th grade are absent. To find how many are present: 90-9=81
Now, for grade 6. There are 80 students, & 15% are absent. I don't have a calculator handy, so here's how I'm doing this (mentally): I'm dividing 80/20, since there are 20 5s in 100. I get 4. Now, to get 15%, we'll multiply 4 by 3 (to reach 15%, since 5•3=15), and we get 12. 12 students are absent in 6th grade, so 68 students are present (80-12=68) And finally, we add both number, 68 & 81, and we find that in both grades, 149 students are present. Hope this wasn't too confusing!
