下面的条形统计图显示了 1 月至 3 月售出的汽车数量。

1. Read the number of cars sold in March from the bar graph

From the graph, the bar for March reaches a height of $9$.
Number of cars sold in March = $9$

2. Calculate the number of cars sold in April using the given fraction

Since March is $\frac{1}{4}$ of April:
$\frac{1}{4}$ of April = $9$
April = $9 \times 4 = 36$

1. Find the number of cars sold in April from part (a)

From part (a), March sold $9$ cars, which is $\frac{1}{4}$ of April.
Number of cars sold in April $= 9 \times 4 = 36$

2. Calculate the total number of cars sold from January to April

Total cars $= 30 \text{ (Jan)} + 18 \text{ (Feb)} + 9 \text{ (Mar)} + 36 \text{ (Apr)} = 93$

3. Calculate the average number of cars sold

Average $= \frac{\text{Total cars}}{\text{Number of months}} = \frac{93}{4} = 23.25$
Since the question asks for the average number of cars, we can leave it as $23.25$ or $23\frac{1}{4}$. (Note: Average can be a decimal even for discrete items).