got two types of info here $ and #buttons
Line them accordingly and then cross multiply
a.
$18/12 dozen buttons =
$ x /4 dozen buttons
X= 4•18/12= $6
b.
$18/12 dozen buttons =
$ x/50 dozen buttons
X=18•50/12= $75
c.
$18/12 dozen buttons =
$27/x dozen buttons
X=27•12/18=18 dozen buttons