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 coal from mine B is to be transported to city N, what 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:
$$(20+8):(24+x)=4: 6$$

That is
$$\begin{array}{l}
24+x=\frac{28 \times 6}{4} \\
x=42-24=18 \text { (yuan) }
\end{array}$$

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