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I am

having trouble figuring out how to answer for Mean, Median, Variance, and Standard Deviation for grouped frequencies.I keep getting a negative for variance which means I cannot calculate standard deviation. Please help me figure out where my errors are.

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You are studying spark plugs in an automobile engine. One aspect of your investigation is the duration of the "spark
burn time" (the duration of the spark) while firing. You test a random sample of 31 spark plugs and obtain the data
displayed in the following histogram.
6
Frequency
4
970
980
990
1000
1030
Spark Time (microseconds)
1010
1020
Q
Use the histogram above to make the following calculations:
Mean:
Median:
Variance:
Standard Deviation:

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