Answer
Total number of seats in the section of stadium = 2869
Work Step by Step
Number of seats in a section of stadium in first Row = 20
Number of seats increase per Row = 3
By formula $a_{n}$ = $a_{1}$ + (n-1)d
Number of seats in $38^{th}$ Row
= 20 + (38 - 1)$\times$3
= 20 + 111
= 131
By formula The sum $S_{n}$ = $\frac{n}{2}$(first term + last term)
Total number of seats in the section of stadium
= $\frac{38}{2}$( 20 + 131)
= 19$\times$151
= 2869