A quarter worth 25 cents and a dime worth 10 cents. It is impossible to have 25 cents since adding dimes we cannot sum to 100 cents. So we can only have 2 quarters. It means that we have 5 dimes that sums to 50 cents. We conclude that we have 5+2 coins, 7 coins. There is 5 dimes and 2 quarters. We deduce the probability that the lost coin is a dime: [tex] \frac{5}{7} [/tex]
