Question 1071618
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A good formula to know for discrete compounding (i.e. annually, quarterly, monthly, daily):
{{{ F = P(1+r/n)^nt }}}

where
    F=future value
    P=present value
    r=annual interest rate (expressed as a decimal)
    n=number of compounding periods per year
    t=number of years

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But for continuous compounding, we need to let {{{n—>(infinity)}}}, and that formula becomes:
   {{{ F=Pe^rt }}}
[ This comes from Lim n—>{{{infinity)}}} {{{((1+1/n)^n) = e }}}, by making the substitution in the discrete compounding formula above of m=n/r  you get n=rm  and you can see the 'e'  part of the limit, raised to the rt power. ] 

    In this problem, F=70000 and we want to know P:

   {{{ 70000 = Pe^((0.04)(18)) }}}
   {{{ 70000 = Pe^0.72 }}}
   {{{ 70000  = P*(2.0544) }}}
   {{{  P = 70000/2.0544 }}}
   {{{ highlight(P=34072.66) }}} dollars