The better deal will be the one that costs the least per diaper. With this in mind, start by calculating the individual diaper cost for both deals.
—30 diapers for $4.25
$4.25/30
≈$0.14 per diaper
—125 diapers for $24.99
$24.99/125
≈$0.20 per diaper
Therefore, the deal with 30 diapers is the better buy.