Problem: An article costing C dollars is sold for $100 at a loss of x percent of the selling price. It is then resold at a profit of x percent of the new selling price S′. If the difference between S′ and C is 191​ dollars, then x is:
Answer Choices:
A. undetermined
B. 980​
C. 10
D. 995​
E. 9100​
Solution:
100=C−x,C=100+x
∴S′=C+191​=100+x+910​=100+100x​(100+x+910​)
∴100x2​+90x​−910​=0∴x=10