Question

A bird flies 61/2 mi in 20 min.

Answer 1

$\frac{61/2}{20} = \frac{x}{81/8}$
multiply the 2nd fraction by 8/8 to get rid of the 8 in the denominator:
$\frac{61/2}{20} = \frac{8x}{81}$
simplify the 1st fraction by multiplying by 2/2:
$\frac{61}{40} = \frac{8x}{81}$
now, you can cross-multiply: 
$61*81=40*8x$
$4941=320x$
$x=\frac{4941}{320}$ or $15 \frac{141}{320}$