milk water problem

A container contains 30 gallons of milk. From this container, 3 gallons of milk was taken out and replaced with 3 gallons of milk. This process is repeated further 2 times. What's the milk to water ratio at the end?

3/30 = 10 pc

30 30-3=27 27(1-10pc) = 24.3 24.3(1-10pc) = 21.87
0 3 3(1-10pc) + 3 = 5.7 5.7(1-10pc) + 3 = 8.13

Answer: 21.87

x gallons milk, replacing with y gallons of water successively n times.

x[(y-x)/x]^n