exam Maths Puzzle Numerical Ability

A random sample of 56 fluorescent light bulbs has a mean life of 645 hours with a standard deviation of 31 hours. Construct a 95% confidence interval for the population mean.

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exam Other Question

this problem is not in the book

I need to solve for x
1/a+1/x=1/b

i know i have to subtract 1/a 1/x=1/b-1/a
but do not know what to do next? What is linear factoring and can you explain on how to do this problem:

*which of the following is a linear factor of 2x^2-x-6?

a. 2x-3
b. x+2
c. 2x+3
d. x-3