Determined the cross elasticity of demand?

When commodity X sold for N25 per unit, 50 units of commodity Y were purchased. With an increase in the price of commodity X to N50 per unit, the demand for commodity Y fell to 20 units.

A. 1.7
B. 0.6
C. -0.6
D. -1.7

Correct Answer:

Option A = 1.7