Step 1: Find the class mark $x_{i}$ of each class and compute $f_{i}x_{i}$.
| Consumption | $f_{i}$ | $x_{i}$ | $f_{i}x_{i}$ |
|---|---|---|---|
| 50-100 | 4 | 75 | 300 |
| 100-150 | 5 | 125 | 625 |
| 150-200 | 13 | 175 | 2275 |
| 200-250 | 20 | 225 | 4500 |
| 250-300 | 14 | 275 | 3850 |
| 300-350 | 8 | 325 | 2600 |
| 350-400 | 4 | 375 | 1500 |
| <b>Total</b> | <b>68</b> | | <b>15650</b> |
Step 2: Total frequency
$$\sum f_{i}=4+5+13+20+14+8+4=68$$
Step 3: Total of the products
$$\sum f_{i}x_{i}=300+625+2275+4500+3850+2600+1500=15650$$
Step 4: Apply the direct method formula.
$$\bar{x}=\frac{\sum f_{i}x_{i}}{\sum f_{i}}$$
Step 5: Substitute the values.
$$\bar{x}=\frac{15650}{68}=230.15$$
Hence the mean monthly consumption is approximately $230.15$ units.