Hi, Hagrid :) Thanks for posting your question here at Brainly. I hope I could help you with this problem.
The incorrect reason would be letter A: add c/a to both sides.
What he did was transpose the term c/a from the left to the right so that he could separate the terms with variables to the left hand side and the constants in the right hand side.
Move the percent two places to the left 1.30
For A divide 2 and 90 for B put 1:and whatever you came up with A. And finnaly for C put 1 x your constant of p
