Given: radius $r = 42$ cm, $\angle AOB = 90°$
Length of arc AB $= \dfrac{\theta}{360} \times 2\pi r = \dfrac{90}{360} \times 2 \times \dfrac{22}{7} \times 42 = 66$ cm
Perimeter of top of table = Arc AB + OA + OB
$= 66 + 42 + 42 = \mathbf{150 \text{ cm}}$
Source: Areas of Sector and Segment of a Circle, Chapter 11
The perimeter of a sector consists of two radii + arc length — students often forget to add the two straight edges (OA and OB). Use the arc length formula $\dfrac{\theta}{360} \times 2\pi r$ with $\theta = 90°$, $r = 42$ cm, and $\pi = \dfrac{22}{7}$.