Hong Kong fixes its exchange rate with the US$ at HK$7.8/US$, but the financial market expects a 10% chance of a 30% devaluation of the HK$ in the coming month and a 90% chance of no devaluation. To maintain the fixed exchange rate at HK$7.8/US$, the interest rate differential between the HK$ and US$ (measured in annual %) needs to be _____________________ %