Elasticity of Demand
Well let's first start off with what Elasticity of Demand is, Elasticity of Demand is a measure of how consumers will react to a change in price.

Something could have either an elastic demand, where E>1 and demand is very sensitive to a change in price, or an inelastic demand where E< 1 and the product is a necessity, or it could be unitary elastic where E=1.

To find the Price Elasticity Demand (PED) three steps must take place:

Step 1
(New Quantity- Old Quantity)/ Old Quantity= % change(delta ) in quantity demanded

Step 2
(New Price- Old Price)/ Old Price= % change ( delta) in price

Step 3
(%(delta) in quantity demanded/ %(delta) in price) = PED

Then you would use the PED to determine the thing's elasticity.