When Tonga's annual income rises from $30,000 to $40,000, she increases her hamburger consumption from 36 to 45 per year, but when her income rises from $40000 to $50000 per year, she decreases her hamburger consumption from 45 to 40 per year, choosing to purchase more steaks instead. Calculate Tonga's income elasticity of demand for an increase in income from $30000 to $40000