Income elasticity is obtained by dividing the percentage change in the quantity demanded of a product with by the percentage change in income.
When income fell by 6 per cent and sales of many fast food restaurants increase by 8 per cent, then the income elasticity for fast food would be:
8/-6 = -1.33
When income fell by 6 percent and sales of soda decreased by 12 percent, then the income elasticity for soda would be
-12/-6=2