Friends can u please help u a bit ??? Can u plz check RD Sharma Solutiuons of Arithmatic Progressions Pg. 9.48  Q's 35 ....and plz explain it.. Friends can u plzz help....

the total amt to be paid is 3600. the person died before paying off 1/3 of the debt which means that he paid 2/3 of the debt. the amt he paid before he died will be 2/3 of his debt that is 2/3 of 3600 which is 2400. sum of first 30 installments that the person paid is given by S30. if  you reduce it you get a value for a. the person had promised to finish paying the debt in 40 installments. this can be given by S40.  if you reduce this you get another value for s. equate both the values of a and you would get an answer for d. substitute this in any one of the equations and you will get the amount paid in the 1st installment.