For a simple points-based test, the percentage is the points earned divided by points possible, multiplied by 100.
42 points out of 50 possible points equals 84%. The letter grade then depends on the grading scale used by the course.
A fixed curve can be represented as an additional percentage-point adjustment when that is how the instructor defines the curve. Other curves use a different method, so check the course rules first.
It varies by school and course, but 60% or 65% is a common minimum passing percentage in many US grading systems. Check the Grading Scale page or your syllabus for the exact cutoff.
Some curves add a fixed number of percentage points to every score; others rescale the whole distribution (for example, setting the top score to 100%). Ask your instructor which method they use before applying an adjustment.
No — both reduce to points earned divided by points possible. For multiple-choice, each correct answer is typically one point; for points-based tests, questions can carry different point values.