Thirteen students do as many push-ups as they can, in a made-up fitness class…

Step 1. The quartiles. Lower half $8, 15, 17, 18, 20, 21$, so $Q_1 = \dfrac{17 + 18}{2} = 17.5$. Upper half $24, 25, 27, 28, 30, 52$, so $Q_3 = \dfrac{27 + 28}{2} = 27.5$. The IQR is 10.

Step 2. The fences. $1.5 \times 10 = 15$. Lower fence $17.5 - 15 = 2.5$. Upper fence $27.5 + 15 = 42.5$.

Step 3. The outliers. Only 52 is beyond a fence. The count of 8 looks low, but it is above 2.5.