Answer:
The forecasted 20% chance of rain represents the probability of rain on any given day. This does not mean that exactly 20% of the days will have rain, but rather each day independently has a 20% chance of rain.
To calculate the expected number of rainy days over the next 7 days, you can multiply the total number of days (7) by the probability of rain on any given day (0.20 or 20%).
So, the expected number of rainy days is 7 * 0.20 = 1.4 days.
This, of course, is a statistical average. In reality, you can't have 1.4 days of rain - you'll either have 1 day, 2 days, or some other whole number of days. But on average, over many sets of 7-day periods, you'd expect about 1.4 days to have rain.