Student: Look at this series: 80, 10, 70, 15, 60, ... What number should come next?
Options:
(A) 20
(B) 23
(C) 25
(D) 30
(E) 35.
Another student: Let's say, hmmm...
This is an alternating addition and subtraction series. In the first pattern, 10 is subtracted from each number to arrive at the next. In the second, 5 is added to each number to arrive at the next. Final answer: (A).