The numbers of letters mailed forms a G.P.: 4, 42, … 48
First term = 4
Common ratio = 4
Number of terms = 8
It is known that the sum of n terms of a G.P. is given by
It is given that the cost to mail one letter is 50 paisa.
Cost of mailing 87380 letters =Rs 87380 x 50/100
= Rs 43690
Thus, the amount spent when 8th set of letter is mailed is Rs 43690.