LCM of 5 and 10 is ___

Find LCM of the $5,10,15,20$

Select the rational numbers which lie between $\frac{2}{5}$ and $\frac{3}{10}$ Use the LCM method.
(A) $\frac{13}{40}$, $\frac{14}{40}$ and $\frac{15}{40}$
(B) $\frac{13}{10}$, $\frac{14}{10}$ and $\frac{15}{10}$
(C) $\frac{13}{20}$, $\frac{14}{20}$ and $\frac{15}{20}$
(D) $\frac{13}{5}$, $\frac{14}{5}$ and $\frac{15}{20}$

Let a = 1 1 1....1 (55 digits), b=1 +10+10${}^2$ +....+10${}^4$, c =1+10${}^5$+10${}^{10}$+10${}^{15}$+....+10${}^{50}$, then