Look at this series: 80, 10, 70, 15, 60, ... What number should come next?

• A. 20
• B. 30
• C. 25
• D. 50

Answer: (A)

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.