Consider that in another month, the number of sunny days is 5 times the number of rainy days. To write this comparison as a ratio, remember that if we consider two quantities a and b, the ratio a: b indicates that there are a units of the first quantity for every b units of the second quantity.
a: b
In our case we have a= 5 sunny days and b= 1, because there are 5 sunny days for every 1 rainy day. This means that the ratio is 5: 1.