Arya has never travelled to another country, but has a collection of foreign coins
given to him by friends and family who have. In his collection he has 10 coins
from Africa, 6 coins from Asia, 7 coins from South America, and 8 coins from
Europe.
One day Arya's grandfather added some Australian coins to the collection. After
he did that, he told Arya that if he took a coin at random from the collection,
the probability of it being from either Africa or Asia was.
How many Australian coins did Arya's grandfather add to the collection?
