将西瓜的质量用最简带分数表示。 答：___ kg

1. Determine the value of one small division on the weighing scale

Between the $2$ kg and $3$ kg marks, there are $5$ equal subdivisions.
Value of one small division = $\frac{3 - 2}{5} = \frac{1}{5}$ kg.

2. Read the mass indicated by the pointer and express it as a mixed number

The pointer is $2$ small divisions past the $3$ kg mark.
Mass = $3 + 2 \times \frac{1}{5} = 3\frac{2}{5}$ kg.
The fraction $\frac{2}{5}$ is already in its simplest form.