Suppose that the -intercept of a line is a (positive) prime number and its -intercept is a positive integer. How many such lines exist that pass through the point
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Let the x-intercept of the line be ( x 1 , 0 ) , and the y-intercept be ( 0 , y 1 ) . Then the slope of the line is − x 1 y 1 and the equation y − y 1 = − x 1 y 1 x . We can substitute the point (4, 3) into this equation to get 3 = − x 1 4 y 1 + y 1 . The rest is nonrigorous casework. Starting from observation when x 1 = 2 , 3 , 5 , etc. we see that the sets ( x 1 , y 1 ) that allow for such line include (5, 15) and (7, 7).