Let common difference of series a1,a2,a3,......,an be d ∵a40=a1+39d=−159 . . . (i) and a100=a1+99d=−399 . . . (ii) From equations (i) and (ii), d=−4 and a1=−3 Since, the common difference of b1,b2,......,bn is 2 more than common difference of a1,a2,......,an ∴ Common difference of b1,b2,b3,..... is (-2) ∵b100=a70 ⇒b1+99(−2)=(−3)+69(−4) ⇒b1=198−279⇒b1=−81