The question is typed wrong.
The correct question should be
171 g sample of tetrafluoromethane (CF4) contains 90 g of carbon. What is the mass percent of fluorine in the compound?
Answer: -
47.37%
Explanation: -
Mass of Carbon = 90 g
Total mass of tetrafluoromethane =171g
Mass of fluorine = Total mass of tetrafluoromethane – mass of carbon
= 171 g – 90 g
= 81 g
Mass percentage of Fluorine = ( Mass of fluorine / Total mass of tetrafluoromethane ) x 100
= (81 / 171) x 100
=47.37%