temperature variation question

The temperature θ2 of a winding

which is being heated electrically at time t is given

by: θ2=θ1(1−e−tτ ) where θ1 is the temperature (in

degrees Celsius) at time t =0 and τ is a constant.

Calculate,

(a) θ1, correct to the nearest degree, when θ2 is

50◦C, t is 30s and τ is 60s

(b) the time t , correct to 1 decimal place, for θ2 to

be half the value of θ1.