The temperature within class $A$ is $20$ C$,$ the outside temperature is $7C.$

After we open all windows of the class, the tamperature of the class decreases to $15$C after $15$ minutes,

a$)$ Find function of cooliing that shows this class's temperature movements.

b$)$ What will the new temperature of the class after another $30$ minutes?

c$)$ When temperature will decline to $10$C$?$