A collection tin holds only one-rupee coins and two-rupee coins, worth 56 rupees altogether. There are 20 one-rupee coins in the tin. One coin is taken out at random. What is the probability that it is a two-rupee coin?

The 20 one-rupee coins are worth 20 rupees, so the two-rupee coins are worth 56 - 20 = 36 rupees, i.e. 36 / 2 = 18 coins. The tin holds 20 + 18 = 38 coins, so $P=\frac{18}{38}=\frac{9}{19}.\frac{10}{19}$ answers for a one-rupee coin; $\frac{9}{28}=\frac{18}{56}$ divides the coin count by the amount of money instead of by the number of coins; $\frac{1}{2}$ assumes equal numbers of each coin.