The topics you should learn are equating denominators and fractions. This is summing up simple fractions.
So we have to make denominators' numbers the same in order to sum up them all (we can sum fractions if the denominators are the same) , and we did it by cross product (that thing on the picture)
Equating by lcm (least common multiple) is the easiest way, because you might see bigger numbers on other questions, and cross product will bring much bigger numbers, so this technic might confuse you.
By the way lcm of 4 and 5 is already 20
Hope it helps!
Answer: it should be 28
Step-by-step explanation: because the way the other problems are solved is the right side of the triangle times the bottom and then adding the left side of the triangle, ( 9x8+7=79 ) ( 5x5+5=30) so to get the answer you’d just multiply 24x3= 72 , then subtract 100-72= 28