Ken's POTW
Rows of Checkers

Place ten checkers into a 4x4 grid to form the largest number of even
rows. An even row has an even number of checkers in it (2 or 4).
Rows may be counted vertically, horizontally, or diagonally.

Place as many checkers as possible into a 6x6 grid, such that there
are no more than two checkers in any straight line, [including all the
diagonal and offdiagonal directions.]
Two checkers have already been placed in opposite corners,
so no more checkers are permitted on that cornertocorner diagonal.

Place sixteen checkers into an 8x8 grid so that there will not be
more than two in a row vertically, horizontally, or diagonally [or any
direction.] There is one stipulation. Two of the checkers must be
placed in the four central squares of the board.
Source:
Mathematical Puzzles of Sam Loyd, Volume Two, edited by Martin
Gardner, Dover Publications, New York, 1960, #155, #84, #48.
Solution
Mail to Ken