4・ 203 There are two coal mines, A and B. Coal from mine A releases 4 calories when burned per gram, and coal from mine B releases 6 calories when burned per gram. The price of coal at the origin is: 20 yuan per ton for mine A, and 24 yuan per ton for mine B. It is known that: the transportation cost of coal from mine A to city N is 8 yuan per ton. If the coal from mine B is to be transported to city N, how much should the transportation cost per ton be to make it more economical than transporting coal from mine A?
[Solution] Let the transportation cost per ton from Coal Mine B to City N be $x$ yuan when the cost is the same as Coal Mine A. According to the problem, we have:
$$
(20+8):(24+x)=4: 6,
$$

which means $24+x=\frac{28 \times 6}{4}$,
$$
x=42-24=18 \text { (yuan) }
$$

When the transportation cost per ton is less than 18 yuan, it is more economical to transport from Coal Mine B compared to Coal Mine A.