2016
Problem - 3004
There is a $40\%$ chance of rain on Saturday and a $30\%$ chance of rain on Sunday. However, it is twice as likely to rain on Sunday if it rains on Saturday than if it does not rain on Saturday. The probability that it rains at least one day this weekend is $\frac{a}{b}$, where $a$ and $b$ are relatively prime positive integers. Find $a+b$.
Let $x$ be the probability that it rains on Sunday given that it doesn't rain on Saturday. We then have $$\dfrac{3}{5}
\cdot x+\dfrac{2}{5}\cdot 2x = \dfrac{3}{10} \implies x=\dfrac{3}{14}$$ Therefore, the probability that it doesn't rain on either day is $$\Big(1-\dfrac{3}{14}\Big)\Big(\dfrac{3}{5}\Big)=\dfrac{33}{70}$$
It follows that the probability that rains on at least one of the days is $1-\dfrac{33}{70}=\dfrac{37}{70}$, so adding up the $2$ numbers, we have $37+70=\boxed{107}$.