Models & Qualitative Behavior · Superchallenge problem

Two identical cups of coffee are poured at $90^\circ$C in a $20^\circ$C room. Each cup obeys Newton's law of cooling, and in $10$ minutes the excess temperature over the room is halved. Into cup A, cream at $5^\circ$C is added immediately; into cup B, the same cream is added after $10$ minutes. The cream is one part to nine parts coffee, and mixing gives the volume-weighted average temperature (same heat capacities). At the $10$-minute mark, right after cup B gets its cream, how many degrees warmer is cup A than cup B?