[กิตติ]

$a_{15}-a_{13} =2d =3 \rightarrow d=\frac{3}{2} $

$a_1+a_2+...+a_m=m(a_1+\frac{d(m-1)}{2} )=325$
$m(4a_1+3(m-1))=1300$...........(1)

$a_1+a_2+...+a_{4m}=4m(a_1+\frac{d(4m-1)}{2} )=4900$
$m(a_1+\frac{d(4m-1)}{2}) = 1225$
$m(4a_1+3(4m-1)) =4900$........(2)

(2)-(1) $3600 = 9m^2$
$m=20$
$8a_1+114=130 \rightarrow a_1 = 2 $

$a_{40}=a_1+39d = 2+39 \times \frac{3}{2}= \frac{121}{2} $