The probability that an event occurs is the number of times it could occur compared to the total number of possible events, assuming the probability of each event occurring is equal.
John has [tex]4+8=12[/tex] coins total. If 4 of the coins are dimes, the probability that he draws a dime is [tex]\frac{4}{12}=\frac{1}{3}\approx 33\% \approx 0.33[/tex].