I already answered it on one of your posts with the same question, but here I'll tell you again :)
Okay so first set up the equation(s) for this situation. In this case, you need two equations:
B=3/4A (In which B= Benjamin and A=Ann)
S=2B (In which S= Siti and B= Benjamin)
Now first solve for the first equation where you can plug in A for 35kg:
B=3/A
B=3/4*35
B=26.25
Now you can use that value and use it for the second equation. Plug in B with 26.25:
S=2B
S=2*26.25
S=52.5
So Siti has a mass of 52.5 kg
