Before adding dissimilar fractions (fractions with different denominator), you have to make the fractions similar first. To make the fractions similar, find the Least Common Multiple of the denominator. In this case, the LCM is 10. Then rewrite the fractions using the LCM as the denominator.
9/10 = 9/10; 4/5 = 8/10
You can now proceed with addition -
9/10 + 8/10 = 17/10 or 1 7/10