The thicknesses in centimeters at the midpoints of 5 equal-width intervals are 19.8, 29.0, 26.9, 20.4, and 9.1. The width of each of the 5 intervals is 50 cm.
An estimate of the area of the cross-section using the midpoint rule.
According to the midpoint rule, the area estimate is the sum of the midpoint values multiplied by the width of the interval. That is ...
... Area ≈ (50 cm)×((19.8 +29.0 +26.9 +20.4 +9.1) cm) = 5,260 cm²