A Wheatstone's bridge is balanced with a resistance of `625Omega` in the third arm, where `P, Q` and `S` are in the `1^(st), 2^(nd)` and `4^(th)` arm respectively. If `P` and `Q` are intercharged, the resistance in the third arm has to be increased by `51Omega` to secure balance. The unknown resistance in the fourth arm is