Clock A loses 2.5 minutes per hour and Clock B gains 1.25 minutes per hour.

Together, they drift apart at a combined rate of:

$2.5 + 1.25 = 3.75 \text{ minutes per hour}$

To differ by exactly 1 hour (60 minutes):

$\text{Time} = \frac{60}{3.75} = 16 \text{ hours}$