first you need to rewrite the fractions 7 and 7/9 as 70/9 and 9 and 2/8 as 74/8. Then you have two add them together (74/8+70/9). in order to do that, you need to make the denominators the same by multiplying 74/8 by 9/9 ( to get 666/72) and 70/9 by 8/8 (to get 560/72). now you have 666/72+560/72 which equals 1226/72. 1226/72 equals 17.028 which is the total amount the class has recycled.
I hope this helps. let me know in the comment if anything is unclear.
