There are 90 students in grade 5, and 10% of them were absent today. in grade 6, 15% of the 80 students were absnet today. what was the total number of students in the two grades who were present today?

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!