I will turn those ratios into fractions and then either cross multiply or isolate the missing variable and solve.
1)
[tex]\frac{10}{5} =\frac{18}{x} \\\\x=\frac{(18)(5)}{(10)}\\\\x=9[/tex]
2)
[tex]\frac{3}{x}\ =\ \frac{27}{45} \\\\x=\frac{(3)(45)}{(27)}\\\\x=5[/tex]
3)
[tex]\frac{x}{5}\ =\ \frac{21}{7} \\\\x=\frac{(21)(5)}{7)}\\\\x=15[/tex]
4)
[tex]\frac{9}{10}\ =\ \frac{x}{20} \\\\x=\frac{(9)(20)}{(10)}\\ \\x=18[/tex]
5)
[tex]\frac{x}{30}\ =\ \frac{20}{10} \\\\x=\frac{(20)(30)}{(10)} \\\\x= 60[/tex]
