How do we approach those type of questions where we have 2 equations in 3 variables and we have to find the possible values? for eg :Karan who had no money in his pocket, presented a cheque for a certain amt to a teller at the bank.The teller intended to give the amt only in the denominations of 50 and 500. He figured that he could give x 50 rupee notes and y 500 rupee notes. However while giving he made an error and gave y 50 rupee notes and x 500 rupee notes. Karan then spent rs 50 and when he counted the remaining amount he found that it was 3 times the amt he had written on the cheque. The amt on the cheque could be :

1) >3000 and <4000
2) >5000 and < 8000
3) >14000 and <16000
4) >16000 and <18000