Chapter 11 - Rotational Dynamics and Static Equilibrium - Problems and Conceptual Exercises - Page 366: 1

$F=60. N$

Torque is equal to $$\tau=rFsin\theta$$ Torque is maximized when $\theta=90^{\circ}$, or $$\tau=rF$$ Solving for $F$ yields $$F=\frac{\tau}{r}$$ Substituting known values of $\tau=15N \times m$ and $r=25cm=0.25m$ yields a force of $$F=\frac{15 N \times m}{0.25m}=60 N$$