posted a question
1. The owner of a computer repair shop has determined that their daily revenue has mean \$7200 and deviation \$1200. The daily revenue totals for the next 30 days will be monitored. What is the probability what the mean daily revenue for the next 30 days will exceed \$7500?
2. The National Association of Realtors estimates that 23% of all homes purchased in 2004 were considered investment properties. If a sample of 800 homes sold in 2004 is obtained what is the probability that between 175 and 200 homes are going to be used as investment property?
3. A new phone system was installed last year to help reduce the expense of personal calls that were being made by employees. Before the new system was installed, the amount being spent on personal calls follows a normal distribution with an average of \$800 per month and a standard deviation of \$50 per month. Find the probability that a randomly selected month had a personal call expense that falls below \$650.
All 3 of these seem to be able to be solved the same way, but I keep getting the wrong answer for all of them. Could you help me understand what to do for these problems?