Please help me with this make up test

How can you decide witch operations to use to slice a two step equation

You are on a cycling trip around europe with friends. you check the weather forecast for the first week. these are the temperatures for the week. sun mon tues wed thurs fri sat 16°c 20°c 24°c 28°c 28°c 23°c 15°c what is the mean temperature forecast for the week?

What is the product? a-3/15a × 5/a-3