$a)$  

${\overline{x}}_d=\frac{60+30+20+10}{4}=\frac{120}{4}=30\left[min\right]$  

${\sigma }_d^2=\frac{{\left(60-30\right)}^2+{\left(30-30\right)}^2+{\left(20-30\right)}^2+{\left(10-30\right)}^2}{4}=$

$=\frac{{30}^2+0^2+{\left(-10\right)}^2+{\left(-20\right)}^2}{4}=\frac{900+0+100+400}{4}=\frac{1400}{4}=350\left[mi{n}^2\right]$ $=\frac{{30}^2+0^2+{\left(-10\right)}^2+{\left(-20\right)}^2}{4}=\frac{900+0+100+400}{4}=\frac{1400}{4}=350\left[mi{n}^2\right]$   

${\sigma }_d=\sqrt{350}\approx 18,71\left[min\right]$  

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${\overline{x}}_c=\frac{30+35+20+35+25+35}{6}=\frac{180}{6}=30\left[min\right]$  

${\sigma }_c^2=\frac{{\left(30-30\right)}^2+{\left(35-30\right)}^2+{\left(20-30\right)}^2+{\left(35-30\right)}^2+{\left(25-30\right)}^2+{\left(35-30\right)}^2}{6}=$  

$=\frac{0^2+5^2+{\left(-10\right)}^2+5^2+{\left(-5\right)}^2+5^2}{6}=\frac{0+25+100+25+25+25}{6}=$

$=\frac{200}{6}=\frac{100}{3}\left[{\text{min}}^2\right]$   

${\sigma }_c=\sqrt{\frac{100}{3}}=\frac{10}{\sqrt{3}}\approx \frac{10}{1,73}\approx 5,78\left[min\right]$  

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  $b)$  

$\overline{x}=\frac{60+30+20+10+30+35+20+35+25+35}{10}=\frac{300}{10}=30\left[min\right]$  

$=\frac{{30}^2+0^2+{\left(-10\right)}^2+{\left(-20\right)}^2+0^2+5^2+{\left(-10\right)}^2+5^2+{\left(-5\right)}^2+5^2}{10}=$

$=\frac{900+0+100+400+0+25+100+25+25+25}{10}=\frac{1600}{10}=160\left[mi{n}^2\right]$   

$\sigma =\sqrt{160}=\sqrt{16}\cdot \sqrt{10}=4\sqrt{10}\approx 4\cdot 3,1622=12,6488\approx 12,65\left[min\right]$