recipe calls for 2 3/4 cups of sugar. if Nora has already put in 1 5/6 cups how many more cups does she need

Step one: go from a mixed number to an improper fraction
2 3/4 = 11/4
1 5/6 = 11/6

step two: make both fractions have the same denominator
4, 8, 12
6, 12
their GCF is 12
11/4 x 3 = 33/12
11/6 x 2 = 22/12

step three: now subtract
33/12 - 22/12 = 11/12

step four: can you simplify the fraction?
no, because 11 is a prime number

final
Nora needs 11/12 of a cup