Question

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?

Answer 1

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!

Answer 2

There would be 81 in grade 5
and 68 in grade 6

because 10% of 90 is 9 so 90 - 9 =81

and 15% of 80 is 12 so 80 - 12 = 68