A whole number is picked at random from 1 to 36. What is the probability that it is NOT a perfect square?

Perfect squares up to 36 are 1, 4, 9, 16, 25 and 36 - six numbers. So numbers that are not perfect squares = 36 - 6 = 30 and $P=\frac{30}{36}=\frac{5}{6}.\frac{1}{6}$ is $P$(perfect square); $\frac{11}{12}$ counts only three squares and forgets 16, 25 and 36; $\frac{2}{3}$ is a guess.