Since every class here has the same width ($h=20$), the deviations from Worked Example 4 can all be divided by 20 to give smaller step values, making the multiplication even easier.

All five classes have the same width, $h = 20$, so the step-deviation method applies: $\bar{x} = A + h\cdot\dfrac{\sum fu}{\sum f}$, where $u = \dfrac{x-A}{h}$, using the same assumed mean $A = 150$ as Worked Example 4.

Step 1 — Compute the step-deviations $u$ and $fu$.

$\sum fu = -16-12+0+10+10 = -8$.

Step 2 — Apply the formula. …

Forgetting to multiply back by $h$ (stopping at $A + \sum fu/\sum f$ instead of $A + h\cdot\sum fu/\sum f$) silently gives a wrong, unreversed answer. The step-deviation method can only be used directly when all class widths …