60. Meteorological observations. At a weather station, it was noticed that during a certain period of time, if it rained in the morning, it was clear in the evening, and if it rained in the evening, it was clear in the morning. In total, there were 9 rainy days, with clear evenings 6 times and clear mornings 7 times. How many days did this entire period cover?
60. There were $\frac{1}{2}(6+7-9)=2$ completely clear days, so the period under consideration covered $9+2=11$ days.

$[$ [M. M., 34, 244 (March 1961).]