Find the price for 1 hotdog:
$140 / 56 = $2.50 per hotdog.
He wants to earn 175 and already earned 140
175 - 140 = 35
He wants to earn $35 more.
Divide the mount he needs to earn by the price of 1 hotdog:
35 / 2.50 = 14
He needs to sell 14 more hotdogs.