Joe's sister's weight gives some momentum to the see-saw, and Joe's force also. These momentums has opposite directions, and for see-saw to be in equilibrium they must be equal in magnitudes.
Momentum's magnitude is the length of the arm (from the center of rotation to the point where force acts) multiplied by the force's magnitude.
If we denote the unknown force as `F,` then
`1 * 300 = 2.5 * F,`
or `F=300/2.5=1200/10=120 (N).`
The answer is 120N.