Five kilograms at two meters creates a positive torque of +98 Nm. Eight kilograms at 1.25 m creates a negative torque of −98 Nm. The heavier mass sits closer to the fulcrum than the lighter mass to create equal and opposite torques that cancel out in equilibrium. This is consistent with what you know about see-saws.
If you have a 3 m long seesaw with a 5 kg weight placed right on top of the fulcrum, where would you put a 4 kg weight to balance it.
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You would have to put the 4 kg weight right on top of the 5 kg weight. A weight right on the fulcrum doesn't produce any torque because its distance to the lever arm is zero. This means that the seesaw is already balanced, so putting the new weight anywhere but right on the fulcrum would unbalance it. 
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