One arm of 325 mm. It's not clear where the 70 kg mass is but I will assume it is at the end of the 325 mm arm. The other arm, at which the force is applied, is 100 mm long. 70 kg mass is a weight of 70g= 70(9.81)= 686.7 N. If the lever were to move through an angle of radians, the mass would swing through a distance , while the force, F, is applied over a distance of . Since "work= force*distance" and work (energy) is conserved, we must have . Now the s cancel and we have F= (325)(686.7)/100= 2231 N. Notice that 2231/686.7= 3.25= 325/100, the ratio of the lengths of the arms.