1/10 of cars in a car park are yellow & another car arrives makes it 1/9 of cars yellow. How many cars now in car park?

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

81 cars of which 9 are yellow.
.
First, there are y cars of which y/10 are yellow.
Now, there are y+1 cars of which (y+1)/9 are yellow.
The new car can be yellow or not. As the fraction of yellow increases, the new car has to be yellow.
.
Thus, y / 10 + 1 = (y+1) / 9.
This solves to y = 80 with 8 yellow cars. With the new yellow car, the parking lot now contains 81 cars.