GMAT - temperature problem

[msquare]

the temperature of a certain cup of coffee 10 minutes after it was poured was 120 degrees fahrenheit. if the temperature f of the coffee t minutes after it was poured can be determined by the formula f=120(2^-at) + 60, where f is in degrees fahrenheit and a is a constant, then the temperature of the coffee 30 minutes after it was poured was how many degrees faherenheit?

[MBARSHAIK]

Ans - 75

First use the given info to get the constant A. It comes out to be 1/10
Next substitute it in teh equation with t = 30.