First, draw a free body diagram.
Our givens :
M = 290kg = Mass of Beam
m = 80kg = mass of Student
L = 10M = length of beam
P = pivot point.
x = length of beam extended.
Then, set up our equilibrium equations.
Tq = Mg*(5-x) - mg * x + F*0 = 0
>> Mg(5-x) = mg*x
Now solve for X
>> 5Mg - xMg = xmg
>> 5Mg = x(M+m)
>> 5M / (M + m) = x
Now plug in your Variables.
5M / (M + m) = x
5(290) / (290 + 80) = x
3.918 = x = distance the beam can extend for a 80kg student to walk to the end.
When you actually do this problem, don't forget the units.
I excluded them above because m for meters would be confusing with the previously defined variables.
Carrying through your units is one of the most effective checking methods there is.