Atest is worth 50 points. multiple- choice questions are worth 1 point, and short answer questions are worth 3 points. if the test has 20 questions, how many multiple-choice questions are there?
a. m + 3s =20
b. m + 3s + 20 =50
c. m + 3s = 50
d. m + s + 50

Correct answer is C. 
m + 3s = 50 
the total number of questions is 20. since the paper is made of only multiple choice and short answer questions, the sum of the multiple choice and short answer questions should be 20.
since multiple choice is 'm' and short answer is 's'
then m + s = 20
but theres no option for that 
if we take the number of points 
points for 1 multiple choice question - 1
then points for m number of multiple choice questions = 1 * m = m
points for 1 short answer - 3
then points for s number of short answer question = 3 * s = 3s
then total number of points = m + 3s
and the total number of points = 50
therefore 
m + 3s = 50
this is the correct answer C.