A subscriber is allowed to make a certain number of telephone calls for a lumpsum charge of Rs.300. Beyond that, he is charged at a certain rate per call. Two subscribers together make 1400 calls, and were charged Rs.425 and Rs.925 respectively. If a single person had made all of the 1400 calls he would have been charged Rs.1550. How many telephone calls are allowed for the first Rs.300?
option
a)300
b) 350
c) 400
d) 450

• Ans is 400;
  
  
  let x call can be made through Rs 300.
  
  then each call cost=(1550-300)/(1400-x)=1250/(1400-x);-------------eq(1);
  
  now
  
  since 1400 call made by two subscriber
  ie
  
  Ist subscriber
  rs 300 for x calls and rs 125 for remaining calls
  
  IInd subscriber
  rs 300 for x calls and rs 625 for reamining calls
  
  
  reaming calls after using calls for rs 300 by both subscriber
  =1400-2x;
  
  each call cost=(rs125(Ist subscriber)+625(IInd subscriber))/(1400-2x);
  =(750)/(1400-2x);-------------------- eq(2);
  
  now equate both equation;
  
  and ans is 400;
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