Each raised bed is 8 feet long, the same as the planks, so the long sides of the beds will each be 1 plank long.
Each raised bed is 2 feet wide, or 2/8 = 1/4 the length of the planks, so the short sides will each be 1/4 plank long.
Each bed is 2 feet tall, and the planks are 1 foot wide, so each side will need 2 pieces of lumber of the same length.
So each bed will require 2 * 2 * 1.25 = 4 * 1.25 = <<2*2*1.25=5>>5 planks of wood.
Bob wants to build 10 raised beds, so he will need 10 * 5 = <<10*5=50>>50 planks of lumber.
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