To cool down coffee, should you add milk now or later

To cool down coffee, should you add milk now or later?
The coffee is at $70^{\circ }$C, the milk is at $10^{\circ }$, the room is at $20^{\circ }$.
(a) Adding 1 part milk to 5 parts coffee makes it $60^{\circ }$. With $y_{\infty }=20^{\circ }$, the white coffee cools to $y(t)=$ ____.
(b) The black coffee cools to $y_c(t)=$ ____. The milk warms to $y_m(t)=$ ____. Mixing at time $t$ gives $(5y_c+y_m)/6=$ ____.

Its source prints no answer for this problem, so there's nothing to check against.

Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) · §6.3 Growth and Decay in Science and Economics, #66