the coffee with the cream added at t = 0 has its initial temperature reduced a fixed number of degrees, however, the coffee w/o the cream added at t = 0 will cool at a faster rate than the coffee with the cream added at t = 0 (why?)
after 5 minutes the cream is added to the second cup which reduces its temperature the fixed number of degrees.
so ... which will have the higher temperature at t = 5 after the cream is added to the second coffee?
in reality, the serious student would conduct a carefully designed and controlled experiment in the middle of the night, drinking the coffee samples to remain awake while analyzing the data. I look forward to your analysis and conclusion.