Answer
384 pounds per square inch
Work Step by Step
Let stress be S,
internal pressure be P
internal diameter of the pipe be D
thickness of the pipe be T
then $S = k\frac{PD}{T}$
S = 100, P = 25, D = 5 and T = 0.75
Putting the values in equation we get
$100 = k \frac{25*5}{0.75}$
=> $k = 0.6$
Find S when P = 40, D = 8 and T = 0.5
From equation
$S = 0.6\frac{40*8}{0.5}$
S = 384 pounds per square inch