Part A. Let the total number of patrons be x. Then the number of patrons that liked reading sci-fi novels is x-35. Now
x-35 is 80%
35 is 100%-80%=20%.
Mathematically, you can write a proportion:
[tex]\dfrac{x-35}{80}=\dfrac{35}{20},\\ \\x-35=\dfrac{35\cdot 80}{20},\\\\x-35=140,\\\\x=175.[/tex]
Part B. The total nuber of patrons is 175, 35 did not like the sci-fi novels, then 140 liked.