Answer:
18.03 inches
Step-by-step explanation:
The cardboard is cut as shown below.
The line c cuts the rectangle into 2 right angled triangles.
To find the diagonal (hypotenuse), we have to apply Pythagoras Rule:
[tex]hyp^2 = opp^2 + adj^2\\\\=> c^2 = 10^2 + 15^2\\\\c^2 = 100 + 225 = 325\\\\[/tex]
=> c = 18.03" = 18.03 inches
The length of c, the diagonal, is 18.03 inches.