How Much Sand Is Needed to Balance a Plank Horizontally?

[iwonde]

Homework Statement

A box of negligible mass rests at the left end of a 2.00-m, 25.0-kg plank (see image). The width of the box is 75.0 cm, and sand is to be distributed uniformly throughout it. The center of gravity of the nonuniform plank is 50.0 cm from the right end. What mass of sand should be put into the box so that the plank balances horizontally on a fulcrum placed just below its midpoint?

Homework Equations

x_cg = (sum of (the masses x position)) / sum of the masses

The Attempt at a Solution

m_p = mass of plank
m_s = mass of sand
x_cg= (m_p(0)+m_s(0.25))/(25+m_s)
I don't know what to set the equation of x-cg equal to. Please help.

 

[Feldoh]

It's equal to whatever position you called the fulcrum