Susan is studying the growth of a particular type of plant. She is testing the effects of four growth factors: lighting, age, temperature, and humidity. Here are the levels she's testing for each.

She wants to find the number of ways of combining one level from each of the growth factors. How many ways are there?

lighting; none, low, medium, high

age: 1 month, 2 months, 3 months

temperature: 20, 22, 24, 26, 28

humidity: low, medium, high