An office used three types of envelopes: small, medium, and large. They used 70 small envelopes. The number of medium envelopes used was 5 times the number of small envelopes plus 5⁄20 of the number of large envelopes

• let small s medium m large l
  given s=70
  m=5s+5/20l
  ---> m=350+5/20l---------eq 1
  l=s+5/20m
  ---> l=70+5/20m----------eq 2
  
  so sub l in eq 1 , m=350+5/20(70+5/20m)
  by solving m=392
  sub m in eq 2 , l=70+5/20(392)
  l=168
  s+m+l =70+392+168 = 630
  
  Ans : 630
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