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The formula for simple interest is I = P * r * t where:
I = interest
r = yearly interest rate. If not time frame is given for the interest rate, such as "annual" or "daily" or "monthly", then yearly is assumed.
t = t in years. 18 months is 1.5 years.
In this case the I, interest earned is:
I = 750*(7.5/100)*1.5 = 84.375 rounded to $84.38