A couple ways to tackle this. You can divide the $281.25 total cost by the 75 posters to get $3.75 per poster. Then multiply the $3.75 per poster by the 100 posters to get a cost of $375.
Another way to do it would be as a ratio:
$281.25/75 = x/100 (where x is the total cost of 100 posters)
multiply both sides by 100
$281.25 * (100/75) = x
$281.25 * 4/3 = X
$375 = x