**(c) 8%**

**Because,**

**Let the required sum be ₹ x**

Rate of interest = r %

Time = 2½ years = 5/2 years

Amount = (6/5) × sum

**Amount = principal + SI**

(6/5) × x = x + [(P × R × T)/100]

= (6/5) x = x + [(x × r × 5)/ (100 × 2)]

= (6/5) = (1 + (r/40))

= r = (40 × (1/5))

= r = 8%