Question

John’s piggy bank contained 7 dimes and 3 quarters. He pulled out 1 coin without looking. Without replacing the first coin, John then pulled out a second coin. What is the probability that both coins were dimes? Express your answer in simplest form. as a fraction

Answer 1

To do this, make a probability tree. The second set of branches are out of 9 as the coin is not replaced. Multiply along the lines in order to work out the probabilities; in a test, multiply along all the lines - even if they are not needed - to ensure full marks. As a check, add up the final values, they should equal 1.