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counting and probability intermediate

Problem

Chubby makes nonstandard checkerboards that have squares on each side. The checkerboards have a black square in every corner and alternate red and black squares along every row and column. How many black squares are there on such a checkerboard?
(A)
(B)
(C)
(D)
Solution
There are 15 rows with 15 black tiles, and 16 rows with 16 black tiles, so the answer is Note: When solving +, you only need to calculate the units digit.
Final answer
B