a)  $1\frac{3}{4}:2\frac{3}{5}:1\frac{1}{2}=$ $\left(20\cdot 1\frac{3}{4}\right):\left(20\cdot 2\frac{3}{5}\right):\left(20\cdot 1\frac{1}{2}\right)=35:52:30$ $\left(20\cdot 1\frac{3}{4}\right):\left(20\cdot 2\frac{3}{5}\right):\left(20\cdot 1\frac{1}{2}\right)=35:52:30$

b)  $2,4:1,2:1\frac{1}{3}=2\frac{2}{5}:1\frac{1}{5}:1\frac{1}{3}=$ $\left(15\cdot 2\frac{2}{5}\right):\left(15\cdot 1\frac{1}{5}\right):\left(15\cdot 1\frac{1}{3}\right)=36:18:20$

c)  $\frac{1}{4}:\frac{1}{5}:\frac{1}{6}=\left(60\cdot \frac{1}{4}\right):\left(60\cdot \frac{1}{5}\right):\left(60\cdot \frac{1}{6}\right)=15:12:10$