Elasticity, link between demand and supply
Example

if p = -1%

and the production increase is +2%

elasticity is -2

[2/-1 = -2]

Our exercise

if this is true

when p = -20%

and production is 60% more

elasticity is -3

[60/-20 = -3]

To sell the 60% more the price has to decrease, if it decreases of 20%, the ratio among products sold (+60%) and price decrease (-20%) is equal to -3.

So, elasticity is the ratio among variation in quantity (60%) and variation in price (-20%) = -3.