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