Question 434253
Surely you must mean 8% interest ANNUALLY, not MONTHLY!  
Assuming it's 8% interest compounded annually, we can write the following formula:
{{{P = P0(1+r)^n}}} 
where P0 is the inital principal, P is the present value, r is the annual interest rate, and n is the period in years.
In this problem, P = 80000, r = 0.08, and n = 18
So we can write {{{80000 = P0(1+0.08)^18)}}}
Solve for P0:
{{{80000/(1+0.08)^18 = P0 = 20020}}}
So the initial investment was $20020
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If the 8% annual interest is compounded monthly, then the rate would be r=0.08/12 and n=18*12
In that case, the initial investment would be:
{{{80000/(1+.08/12)^216}}} = $19045