9.1. Ivan wanted to buy nails. In one store, where 100 g of nails cost 180 rubles, he could not buy the necessary amount because he was short 1430 rubles. Then he went to another store, where 100 g cost 120 rubles. He bought the required amount and received 490 rubles in change. How many kilograms of nails did Ivan buy?
Answer: 3.2 kg.

According to the problem, the price difference is 60 rubles per 100 g, or 600 rubles per kilogram. If this is multiplied by the mass of the nails in kg, we get $1430+490=1920$. Therefore, Ivan bought $1920: 600=3.2$ (kg).