**Mean (Step-deviation method):** $a=45$, $h=10$.

| Class | Midpoint | $f_i$ | $u_i$ | $f_iu_i$ |
|---|---|---|---|---|
|10-20|15|5|-3|-15|
|20-30|25|4|-2|-8|
|30-40|35|10|-1|-10|
|40-50|45|13|0|0|
|50-60|55|12|1|12|
|60-70|65|10|2|20|
|70-80|75|6|3|18|

$N=60$, $\Sigma f_iu_i=17$
$$\text{Mean}=45+10\times\frac{17}{60}=45+2.83=47.83$$

**Mode:** Modal class = 40-50 (highest frequency 13). $l=40,\ f_1=13,\ f_0=10,\ f_2=12,\ h=10$
$$Mode=40+\left(\frac{13-10}{26-10-12}\right)\times10=40+\frac34\times10=47.5$$