Question

The high temperature x (in degrees centigrade) during winter-time in washington

Answer 1

A suitable probability calculator can tell you the desired temperature is about 26.2 °C.

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The invNorm function on this calculator gives the x-value below which the probability distribution has the given area. Since you want the x-value for the area above being 0.3, the area below that x-value will be 1-0.3 = 0.7.