Answer
$a_{1} = 3, d=5$
Work Step by Step
$a_{n}$ of the arithmetic sequence is $a_{n} = a_{1} + (n-1)d$
Given $a_{4} =18$
$a_{20} = 98$
$a_{4} =a_{1} + (4-1)d$
$a_{4} =a_{1} +3d$
$a_{1} + 3d = 18$ Equation $(1)$
Similarly,
$a_{20} =a_{1} + (20-1)d$
$a_{20} =a_{1} +19d$
$a_{1} +19d= 98$ Equation $(2)$
Subtract Equation $(1)$ from Equation $(2)$
$a_{1} + 19d - (a_{1} + 3d)= 98-18$
$a_{1} + 19d - a_{1} - 3d= 98-18$
$16d = 80$
$d = 5$
Substituting $d$ value in Equation $(1)$
$a_{1} + 3d = 18$
$a_{1} + 3(5) = 18$
$a_{1} + 15 = 18$
$a_{1} = 3$
