Please solve the calculation

I agree completely. But the applicant has considered the mass and not the strength.

Robert;

You're getting confused by the way Mathcad handles units.

There is a difference between lb and lbf; lb is a mass unit (like slugs and kilograms.)  lbf is a force (like Newtons.)  Your equations need to be unit balanced if they're correct.  Now, you define Q as two terms, the first term has defined units:

So Q has units of lbf/in.

When you write lbs/in of circumference, you're specifying force per unit length.  To Mathcad, that's lbf/in, or N/m, or . . .