Efficiency vs frequency

Efficiency (%)	82	83	84	85	86	87	88	89	90
Frequency	1	2	2	3	4	4	6	2	1

Making use of the data generated, interpret the data into meaningful information:

1. Calculate the arithmetic mean efficiency.
2. Calculate the standard deviation showing all the important steps.
3. The probability that the machine's efficiency will fall between 85% and 88%.