No, you wouldn't do 25% because the last 15% is 15% of the 90% of the original. In mathematical terms:

T(x) = T(0)(1-0.1)(1-0.15) where T(x) is the discounted price and T(0) is the original price. (1-0.1) is the original 10% off and (1-0.15) is the further 15% off

We know T(x) = 306 and need to find T(0)