Raffle coupons numbered 100 to 199 are put into a drum and one coupon is pulled out at random. What is the probability that the number on it is divisible by 9?

The coupons 100 to 199 make 100 equally likely outcomes. Multiples of 9 in that range are 108, 117, 126, 135, 144, 153, 162, 171, 180, 189 and 198 - eleven of them. $P=\frac{11}{100}.\frac{1}{9}$ assumes exactly one number in every nine falls in the range, which needs the count to be a multiple of 9; $\frac{3}{25}=\frac{12}{100}$ counts 99 or 207 by mistake; $\frac{1}{10}$ confuses this with the multiples of 10.