x=P/A=250/(1+0.2sec(50√(x/20000))) after substitution of known quantities.
If x=20 on the right-hand side, x (left-hand side)=208.3201087.
Substituting this in the formula: x=208.1952558, then substituting again, x=208.1953387 which is the stable result. This is the fixed-point method.
The bisection method doesn’t seem appropriate in this case, and the solution is P/A=208.1953 to 4 decimal places.