The common factor would be 3 because 3 is the highest multiple of 15 and 9
Its 110 cuz it goes up 10% if it was 20% it would be 120
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!
You have to multiply 4 decimal 5 * 2 and you get your answer and 0.7 * 2 and then you add up those two answers such a bit and see what you get
Her answer is wrong because the product of 78.112 = 8736.