Let "a number" = x
3x - 5 = 40 is the equation you will use.
Note the equal sign, what you do to one side, you do to the other. Do the opposite of PEMDAS.
First, add 5 to both sides
3x - 5 (+5) = 40 (+5)
3x = 40 + 5
3x = 45
Next, to isolate the x, divide 3 from both sides
(3x)/3 = (45)/3
x = 45/3
x = 15
15 is your answer
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