Question

how many cubic yards of concrete will be needed to replace a section of sidewalk 150ft long, 5ft wide and 4 inches thick

Answer 1

4 inches = 1/3 foot

So the volume of that section is  (150ft x 5ft x 1/3 ft) = 250 cubic feet

The whole trick to this question is knowing
how many cubic feet make 1 cubic yard.

1 yard = 3 ft

 (1yd x 1 yd x 1yd) = 1 yd³

(3ft x 3ft x 3ft) = (3 ft)³ = (3)³ x (ft)³  =  27 ft³    <===

For the sidewalk project,

               250 ft³  =  (250 / 27)  =  9.26 yard³ .

I think the way concrete contractors operate, they'll
want you to order and pay for 10 cubic yards.