You need the molercular mass of Sucrose.
You can either search the molecular formula of Sucrose and calculate the molecular mass, or search directly the molecular mass.
Here you have both:
Molecular mass of sucrose: 342.30 g / mol
Chemical formula of sucrose: C12 H22 O11 (if you want to just must multiply the number of each atom by its atomic mass to get the molecular mass).
Calculate the number of grams of O in that molecule:
O: 11 * 16.00 g/mol = 176.0 g of O.
Now use proportions:
176 g O / 342.30 g sucrose = x / 50.00 g sucrose
=> x = 50.00 g sucrose * 176.0 g O / 342.30 g sucrose =25.71 g O.
Answer: 25.17 g