P,Q,R and S are the resistances taken in the cyclic order in a wheatstone bridge network. P and Q are ratio coils, S is the unknown resistance and R is a `20Omega` coil. A balance is obtained when R is shunted with a resistance of `350Omega`. When P and Q are interchanged, the balance is restored by altering the shunt across R to `498Omega`. find the resistance of S and the ratio P:Q.