In a certain town, the probability that it will rain in the afternoon is known to be $0.6$ Moreover, meteorological data indicates that if the temperature at noon is less than or equal to $ 25°C$, the probability that it will rain in the afternoon is $ 0.4$. The temperature at noon is equally likely to be above$ 25°C$, or at/below $25°C$. What is the probability that it will rain in the afternoon on a day when the temperature at noon is above $25°C$\_\_\_\_?

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My attempt:

$\implies P(\text{rain in afternoon}) = 0.5 \times P(\times{rain when temp \leq25}) + 0.5 \times P(\text{ rain when temp>25})$,

$\implies 0.6 = 0.5\times 0.4 + 0.5\times P( \text{rain when temp>25})$

So,  

$P( \text{rain when temp>25}) = 0.8$

I am using the rule $P(R)=(1/2)\times P(A)+(1/2)\times P(B)$ , I applied it , but I have no clear point , why I gives the priority to equal , why I don't give the chances to not equal $(i.e. P(R)=(2/3)\times P(A)+(1/3)\times P(B)$ ).

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