Answer:
17
Step-by-step explanation:
all 4 numbers added up, 68, divide by the amount of numbers, which is 4, and you get the answer
13,15,20,20
add all of the numbers and divide by how many numbers there are.
13+15= 28
20+20= 40
28+40=68
68
68/4= 17 ,since there are 4 four numbers
Answer : 17
