For routing of flood in a given channel using the Muskingum method, two of the routing coefficients are estimated as $\style{font-family:'Times New Roman'}{C_0=-0.25\;and\;C_1=0.55}$. The value of the third coefficient $C_2$ would be ______

A culvert is designed for a flood frequency of 100 years and a useful life of 20 years. The risk involved in the design of the culvert (in percentage, up to two decimal places) is ______

A 1-h rainfall of 10 cm magnitude at a station has a return period of 50 years. The probability that a 1-h rainfall of magnitude 10 cm or more will occur in each oftwo successive years is: