When adding or subtracting fractions we need one very important thing: Common denominators.
This means that the bottom numbers must be the same for both fractions. We achieve this by multiplying the denominators to a number that both of the denominators multiply to. It can't just be any multiple of the two it must be the lowest. This is called the Least Common Denominator.
In this case the LCD is 28. The 4 divides into 28 seven times and the 7 divides into 28 four times. This means we have to multiply the numerator by the same number we multiplied the denominator by to achieve this number. The 1 in 1/4 gets multiplied by 7 and the 2 in 2/7 gets multiplied by 4.
This gives us 7/28 + 8/28 . This equals 15/28
15/28
is the answer you find the least common denominator (28) then you combine
