Question 1059726
Let D be the 10 cent coin, let Q be the 25 cent coin.
Look at the possible outcomes of a two coin draw and the sums,
DD=>20
DQ=>35
QD=>35
QQ=>50
Look at the probability of each outcome (I wont simplify until the final answer),
DD:{{{P=(5/9)(4/8)=20/72}}}
DQ:{{{P=(5/9)(4/8)=20/72}}}
QD:{{{P=(4/9)(5/9)=20/72}}}
QQ:{{{P=(4/9)(3/8)=12/72}}}
When you sum the probabilities, they add up to 1 so all possible outcomes are covered.
So for at least 35, sum the probabilities where the sum is at least 35.
{{{P=(20+20+12)/72=52/72=13/18}}}