QUESTION $1[12$ marks$]$

The water level in a farm dam is automatically regulated by a system as shown in Figure $1$ below $($sectional elevation$).$ The gate has a length of $115$ cm and a width of $60$ cm $.$ The homogenous prismatic gate, with a mass of $125$ kg $($excluding the lever arm and the counter$-$weight$),$ hinges at point O$.$ Determine the mass of $W($the counter$-$weight at the end of the lever arm$)$ if the gate is at the point of opening when the water level is $130$ cm above the pivot point. Ignore the weight of the lever arm.