The production function of good X is Qx = 5 Lx, and Lx decreases by 1/10. We will therefore have:
dQx = 5 dLx = 5 (- 1/10) = - (5/10) = - (1/2)
The difference (d) in Qx is 1/2 unit.
It should not be surprising that the opportunity cost of Y is the reciprocal of the opportunity cost of X -- to produce one more unit of X you give up two units of Y; to produce one more unit of Y you give up one-half unit of X.