S = $\frac{5^2}{1.6}$ + $\frac{5^2}{6.11}$ + $\frac{5^2}{11.16}$ + $\frac{5^2}{16.21}$ + $\frac{5^2}{21.26}$
⇔ S = 5($\frac{5}{1.6}$ + $\frac{5}{6.11}$ + $\frac{5}{11.16}$ + $\frac{5}{16.21}$ + $\frac{5}{21.26)}$
⇔ S = 5($\frac{6-1}{1.6}$ + $\frac{11-6}{6.11}$ + $\frac{16-11}{11.16}$ + $\frac{21-16}{16.21}$ + $\frac{26-21}{21.26}$)
⇔ S = 5($\frac{6}{1.6}$ - $\frac{1}{1.6}$ + $\frac{11}{6.11}$ - $\frac{6}{6.11}$ + $\frac{16}{11.16}$ - $\frac{11}{11.16}$ + $\frac{21}{16.21}$ - $\frac{16}{16.21}$ + $\frac{26}{21.26}$ - $\frac{21}{21.26}$)
⇔ S = 5(1 - $\frac{1}{6}$ + $\frac{1}{6}$ - $\frac{1}{11}$ + $\frac{1}{11}$ - $\frac{1}{16}$ + $\frac{1}{16}$ - $\frac{1}{21}$ + $\frac{1}{21}$ - $\frac{1}{26}$)
⇔ S = 5(1 - $\frac{1}{26}$)
⇔ S = 5 × $\frac{25}{26}$
⇔ S = $\frac{125}{26}$
Vậy S = $\frac{125}{26}$
