the elasticity of demand for salt ​is zero. if the demand is 2 kg the price of rs 5 per kg,calculate  the demand , if the price rises to rs 7.50 per kg?

Since the demand is perfectly inelastic (Ed = 0), a change in price does not cause the quantity demanded to change. Thus, if the price rises to Rs 7.5 per kg, the quantity demanded of salt remains 2 kg.