Question 1163402
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It is a classic Ordinary Annuity saving plan. The general formula is 


    FV = {{{P*(((1+r)^n-1)/r)}}},    (1)


where  FV is the future value of the account;  P is the monthly payment (deposit); r is the monthly percentage yield 
presented as a decimal; n is the number of deposits (= the number of years multiplied by 12, in this case).


Under the given conditions, P = 400;  r = 0.07/12;  n = 12*20 = 240.  So, according to the formula (1), you get 
at the end of the 20-th year


    FV = {{{400*(((1+0.07/12)^(12*20)-1)/((0.07/12)))}}} = {{{400*(((1+0.07/12)^240-1)/((0.07/12)))}}} = $208,370.70.


Note that you deposit only  12*20*$400 = $96,000.  
The rest  $208370.70 - $96000 = $1212370.70  is the interest what the account earns/accumulates in 20 years.
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On Ordinary Annuity saving plans, &nbsp;see the lessons

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/Sequences-and-series/Ordinary-Annuity-saving-plans-and-geometric-progressions.lesson>Ordinary Annuity saving plans and geometric progressions</A>

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/Sequences-and-series/Solved-problem-on-Ordinary-Annuity-saving-plans.lesson>Solved problems on Ordinary Annuity saving plans</A>

in this site.


The lessons contain &nbsp;EVERYTHING &nbsp;you need to know about this subject, &nbsp;in clear and compact form.


When you learn from these lessons, &nbsp;you will be able to do similar calculations in semi-automatic mode.