Store and reuse intermediate variable

[Thomas Klemm]

You could also just duplicate the value with DUP before creating the local variable R:

Code:

« "Ins C t A n" {
":C:
:t:
:A:
:n:
" { 1 0 } V } INPUT OBJ→
→ C t A n
  « 'C*t/100/n/(1-(1+t/100/n)^(-n*A))'
    →NUM "R" →TAG DUP → R
    'R*A*12' →NUM "M" →TAG
  »
»

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Just noted that you are now using → NUM instead of →NUM in your listing.
That could be the reason why you got an algebraic result.