11. A two-digit number, when 5 is written in front of it, the resulting three-digit number is 12 less than 9 times the original two-digit number. The original two-digit number is ( ).
【Analysis】If we write 5 in front of this two-digit number and then add 12, the total increase is 512. At this point, the resulting number is exactly 9 times the original number, meaning it has increased by 8 times. Therefore, the original two-digit number is $512 \div 8=64$.