A sampling grew 5 1/2 inches one year 6 3/4 inches the second year,and 7 1/4 inches the third year.how much did it grow during the three years

Essentially we are adding these three mixed numbers: 

5 1/2 + 6 3/4 + 7 1/4 

the common denominator is 4, so we convert: 

5 2/4 + 6 3/4 + 7 1/4 = 
5+6+7 + 2/4+3/4+1/4 = 
18 6/4= 
18 + 4/4 +2/4= 
19 2/4= 
19 1/2