We are given the following details:
Small cup of Coffee = $2
Regular cup of Coffee = $4
Large cup of Coffee = $5
1.
Jon orders 4 fewer large cups than David as shown below. The amount he spent is shown below:

This means that Jon spent $30 on coffee
2.
The equation is shown below:

3.
Jon bought zero small cups of coffee. This is represented as:

Hence, Jamie is correct
4.
David ordered 10 large cups of coffee as shown below:

If David paid for his order with a $20 bill, we have:

That implies that $20 cannot cover David's order, he'd still be having a deficit of $30