Grinch's Toy Store ???

Grinch's Toy Store

After disappointing Black Friday sales, management at Grinch's Toy Store found a way to boot profits. Following an approach used in some amusement parks, GTS now sells coupons, which customers then use for payment at checkout. The store's inventory comprises 22 toy models, each clearly marked with a unique, whole-dollar price. Coupons are available in $6, $9 and $20 denominations. Grinch's strategy is threefold.

[1] At checkout, extra coupon value is forfeit; no change is given. Example: your total at checkout is $5. You pay with a $6 coupon. Grinch's keeps the $1 change.

[2] Prices have been set to ensure there is extra coupon value when toys are bought singly.

[3] Each toy model is limited one to a customer.

Your task is to visit Grinch's, browse the shelves, and find the minimum number of toys which, when bought together, using coupons, will ensure a fair purchase at checkout.That is, there will be no change for Grinch's to keep.Grinch's Toy Store ???
Great way to make more money!
what??

i don't get it is this a math problem

if so someone explainGrinch's Toy Store ???
I don't get it... plz help.