*y = ax + c*,

*y*

*=*

*ax + d*,

*y = bx + c*, and

*y = bx + d*has area 18. The parallelogram bounded by the lines

*y = ax + c*,

*y = ax -*d,

*y = bx + c*, and

*y = bx - d*has area 72. Given that

*a, b, c,*and

*d*are positive integers, what is the smallest possible value of

*a + b + c + d*?

Interactive version of problem and solution.