F = \(\frac{uv}{u + v}\) F = +ve v = -ve 30 = \(\frac {25 {\times} (-v)}{25 + (-v)}\) -25v = 750 - 30v 5v = 750 v = 150cm (Note that either negative or positive values go to show the positioning and hence, they are not a strong necessity in your final answer.)