Jessica has a die with 8 sides, another with 15 sides, and a third one with 5 sides. She rolls the three dice once. Let Z be the number of dice landing on the number one. Find the expectation of Z using the indicator method.

Note. If the three numbers are 1, 11, 3 then Z is equal to 1. Provide a numerical answer with precision up to 3 decimal place

Answer is 0.392. But I don't understand why