A cottage stands surrounded by an orchard of fifty-five trees. 
Ten of these trees are cherries, ten are plums, and the remainder apples. 
The cherries are so planted as to form five straight lines, with four cherry trees in every line. 
The plum trees are also planted so as to form five straight lines with four plum trees in every line. 

Which are the ten cherry trees and which are the ten plums?

In order that the cherries and plums should have the most favourable aspect, as few as possible (under the conditions) are planted on the north and east sides of the orchard. 
Of course in picking out a group of ten trees (cherry or plum, as the case may be) you ignore all intervening trees. 
Four trees may be in a straight line irrespective of other trees (or the house) being in between.

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