In a certain supermarket, a triangular display of cans is arranged in 10 rows, numbered 1 through 10 from top to bottom. Each successively numbered row contains 3 more cans than the row immediately above it. If there are fewer than 150 cans in the entire display, how many cans are in the fifth row?
Answers.
A) 10 B) 11 C) 13 D) 14 E) 16
Let x be the number of cans in row 1.   The total number of cans is x + (x+3) + ... + (x+27) =   10x + 3(1+2+...+9) =   10x + 3(9)(10)/2 =   10x + 135   Since the total is less than 150, x must equal 1.   The number of cans in the 5th row is 1 + 3(4) = 13   The answer is 13.
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In a certain supermarket, a triangular display of cans is arranged in 10 rows, numbered 1 through 10 from top to bottom. Each successively numbered row contains 3 more cans than the row immediately above it. If there are fewer than 150 cans in the entire display, how many cans are in the fifth row?