Question

If the price of jelly goes up by 10 percent, we observe a decrease in the quantity demanded of peanut butter of 20 percent. the cross-price elasticity of these goods is:

Answer 1

Cross price elasticity refers to the measure of responsiveness of the quantity demanded of a product to a change in price of another good. 
From the question given above, 
cross price elasticity = -20% / 10% = -2.
The cross price elasticity for the goods above is - 2. Which means that the goods are not substitutes. 
A positive cross price elasticity which is greater than zero means that the goods are substitutes.