<span>Probability is usually calculated as the number of ways an event has occurred over the overall number of outcome. Theoretical probability is therefore the probability where an event is expected to happen whereas experimental probability is an event that certainly happens.</span>
First you had to subtract by seventeen from both sides of equation form.

Then simplify.

Next, subtract by seven-p from both sides of equation form.

Simplify.

Then you divide by negative six from both sides of equation form.

And finally, simplify by equation.

Final answer: 
Hope this helps!
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-Charlie
85/100 - 17/20 simplified
Most graphing calculators will do weighted averages pretty easily. It is mostly a matter of data entry.
mx = -2
my = 10
(x, y) = (mx, my)/10 = (-0.2, 1)