LET us say that volume of mercury poured in vessel bev.

so vol of the air initially is 1000 - v        -----------------equ.1

let the temperature is raised by dt  so final volume of vessel will be 1000(1+1*dt) ------------------------equ.2

and final volume of mercury is v(1+2*dt)----------------------eqa 3

where         are coeff of vol exp. of vessel and mercury respectively

now as given in question the vol of air doed not changed so eqa 1=eqa2  --  eqa 3

solving which we finally get  

which gives v=50 cc