Monthly Demand Elasticity

Monthly demand for juice by consumer is given by Q=10-8P

a. If the price of juice is $1 per can, how much/many juice will the consumer purchase in a typical month?

Here I just substituted $1 in for P, so the quantity demand at $1 for juice would be Q=2

b. What is the elasticity of demand for juice?
i have the formula to use %change Qd/%change P however note sure where these are coming from since don't know the change unless plug in for various Q and P variables?

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