Converting to a common denominator: 1/4 = 5/20 and 3/5 =12/20 so the sum would be (5+12)/20 = 17/20
Alternatively you could convert the fractions to decimal: 1/4 = 0.25 and 3/5 = 0.6 so the sum would be 0.25 + 0.6 = 0.85 (which - if you reduce to a fraction again - is, of course, 17/20)