In a stair-like decoration the bottom row has (60) bulbs and each upper row has (5) fewer bulbs. How many bulbs are in the (9)th row?

Correct answer: (20)

The direct answer is option B, 20 bulbs. The rows form an arithmetic progression because the same number is added or subtracted each time. The first row has 60 bulbs, and moving upward reduces the number by 5, so the common difference is \(d=-5\). The ninth row is \(a_9=a+(9-1)d=60+8(-5)=60-40=20\). Therefore B is correct. Option A, 15, would result from subtracting 5 nine times instead of only eight times after the first row. Option B, 20, matches the calculation. Option C, 25, is too large because it subtracts only 35. Option D, 30, is also too large because it subtracts only 30. Remember: the first row is already counted, so the ninth term has eight changes.