(2) According to the formula (1),180 ≤10x+150 ≤ 200.

It is found that 3≤x≤5 means that there are three car purchase schemes, namely, 3 large buses and 7 medium buses, and the car purchase cost is 3× 25+15× 7 =175; There are 4 large buses and 6 medium buses, and the purchase cost is 25× 4+15× 6 =180; There are 5 large buses and 5 medium buses, and the purchase cost is 25×5+ 15×5=200.

Therefore, the car purchase scheme of four large buses and six medium-sized buses makes the car purchase cost the smallest.