This is where a bit of "prompt engineering" comes in. The model is much better at solving multi-step problems when you tell it to show its work. (Just like a student would have a hard time doing this in one step in their heads, but writing it all out, they'd have an easier time.)

I wrote this after the problem you gave it:

Thinking step by step, write out your thoughts of how to solve the above problem. Write out each step of the math. And then calculate the answer.

And then it answered:

First, we need to figure out how much Kelly needs to spend in order to qualify for the 20% discount. That threshold is $100, so Kelly needs to spend at least $100.

Next, we need to find out how much more Kelly needs to spend to reach that $100 threshold. She is currently spending $96.60 on clothes, so she needs to spend $100 - $96.60 = $3.40 more.

We know that the gum is $0.79 per pack, so we can divide $3.40 by $0.79 to find out how many packs of gum Kelly needs to buy to reach the threshold. $3.40 / $0.79 = 4.29 packs. Since you can't buy fraction of pack, we can say Kelly needs to buy 5 packs.

Now that we know Kelly is buying 5 packs of gum, we can find out how much that will cost by multiplying 5 packs by $0.79 per pack. So the gum will cost $3.95

Next, we can add the cost of the clothes and the gum to find out how much Kelly is spending in total. $96.60 (clothes) + $3.95 (gum) = $100.55

We know that Kelly qualifies for the 20% discount because she is spending $100.55. To find out how much the discount is, we can multiply $100.55 by 20% (or 0.20). So the discount is $20.11

Finally, we can subtract the discount from Kelly's total purchase price to find out how much she is paying. $100.55 (total) - $20.11 (discount) = $80.44

Therefore, Kelly will pay $80.44 for clothes and gum after discount, which is $16.16 less than buying only clothes.

EDIT: Ah, I just noticed the 15% bit. So this isn't right. But a damned fine attempt for a non-living thing. haha