Give the step-by-step reasoning process and then the final answer. A bicycle shop owner adds 3 bikes to her stock every week. After 1 month, she had sold 18 bikes but still had 45 bikes in stock. How many bikes did she have originally?
There are approximately 4 weeks in a month. The shop owner added 3 bikes every week for 4 weeks for a total addition of 3 * 4 = 12 bikes. The shop owner now has the original number of bikes + 12 bikes. She sold 18 bikes within the same time frame. She now has a total of 45 bikes which is: the original number of bikes + 12 - 18 = 45 bikes. The original number of bikes = 45 - 12 + 18 = 51 bikes.
The final answer: 51.