Putting the two box-and-whisker plots side-by-side on the same scale (which is the main purpose of drawing these plots in the 1st place as they're very good at providing an easy visual comparison of different sets of data):

When presented with the above, our foreign friend Boon realizes in a nanosecond that he should stick to Bus Service A because:

1. Service A has shorter waiting times in general (due to lower median), even though there's a small chance of Service B arriving earlier than Service A due to its lower minimum value.
2. Service A's waiting times are less variable and more predictable compared to Service B (Service A has a lower range, lower interquartile range ⇒ less spread).

An excited Boon says "KHAWP KHUN KHRAP!" and may he finally get to hear the verse "... Semua kita berseru!" soon!

We line-up all the terms in the stem-and-leaf diagram like prisoners in ascending order.

As the total number of terms here is 20 (an even number), we'll need to take the average of the two middle terms i.e. the 10th (T₁₀) and 11th (T₁₁) terms as the median (Q₂).

Similarly we need to take the averages of T₅/T₆ and T₁₅/T₁₆ for the lower quartile (Q₁) and upper quartile (Q₃) respectively, since there are no terms that lie exactly at the 25% and 75% mark in the line of numbers.

No problems with your answers here *clap clap*

P.S. How is the stem-leaf diagram different from the ones you have?

You first need to know where your Min/Max/Q_{1 ... 3} values on the cumulative frequency curve are:

and then map them to your box-and-whisker diagram:

where you got most of them right except for x₁ and x₄.

In cumulative frequency questions, tragedies often occur at the axes and the extreme ends of the curve.

Do you know that Miss Loi has ... umm ... cheekily extended the y-axis to 24 when the curve only ends at 20? Instead of looking at where the curve ends, some students (in their haste) are bound to erroneously assume that the total number of buses = 24 and use this to obtain their Q₁/Q₂/Q₃.

While you managed to survive that, you were unfortunately caught out at the lower end of the curve - the minimum value (i.e. x₁) cannot be zero as the curve starts at 30 (i.e. the lowest waiting time = 30 mins as none of the buses arrived in ≤ 30 mins).

Due to this, your range should be 46-30 = 16mins instead.

Also, not sure how you obtained x₄ = 41 (careless reading of the graph?). This in turn affected your interquartile range which should've been 40-34 = 6 instead.

So one needs to first get acquainted with the main parts of the plots:

P.S. Note the difference between range and interquartile range.

And since they're just another representation of the data that you have ...

Iris? You mean this Iris???

Please don't sabo our dear foreign friend here (esp if he needs to take Service 61)!

P.S. Your stem-and-leaf plot is very different to the ones I see at my school. Just a question: what grades would "E-Maths" be equivalent to, internationally?