Problem related to the transfer function of RC low pass filter
The transfer function of this circuit is: 
$$H(i\omega)=\frac{10}{10-i\omega}$$

Insert w=1 and w=100 into the transfer function: 
$$H(\omega=1)=\frac{10}{10-i}$$
$$H(\omega=100)=\frac{10}{10-100i}$$

Build the absolute ratio of both transfer functions:
$$\displaystyle \left|\frac{H(w=1)}{H(w=100)}\right|$$
$$= \displaystyle \left|\frac{10}{10-i}*\frac{10-100i}{10}\right|$$
$$= \displaystyle \left|\frac{10-100i}{10-i}\right|$$
$$= \displaystyle \left|10*\frac{1-10i}{10-i}\right|$$
$$= 10$$