Question

One-tenth of the cars in a car park are yellow. Another car arrives and now one-ninth of the cars are yellow. How many cars are now in the car park?

Answer 1

1/10 of cars are yellow in the car park
9/80
+1 = 81
9/81 = 1/9
there is a total of 81 cars after the new car arrives

Answer 2

There are 81 cars now in the car park! :D