在数轴上，X 代表的值是多少？

1. Find the value of one interval on the number line

The difference between $4.0$ and $3.9$ is $4.0 - 3.9 = 0.1$.
There are $5$ equal intervals between $3.9$ and $4.0$.
Value of $1$ interval $= 0.1 \div 5 = 0.02$.

2. Calculate the value represented by X

X is located $3$ intervals to the right of $4.0$.
Value of X $= 4.0 + 3 \times 0.02 = 4.0 + 0.06 = 4.06$.