24 hours passed.

Answer is $1.10

I started with $7 - $3.80 for the noodles leaving $3.20.
From $3.20, after coffee I still have $2.10 remaining.
Therefore $3.20-$2.10 = $1.10 which is the price of the coffee.
Now, let me do so curating and get back to figuring out who to pay. ;-)
Sorry, I've been quiet on this post to be fair to everyone.

24小时过了。

答案是$1.10

我开始有$7。然后，用$3.80买了面剩余$3.20.
从$3.20再买杯咖啡，仍然盛$2.10。
因此$3.20-$2.10等于$1.10-就是咖啡的价格。
现在，让我看看别帖后再来搞清楚奖金的账。;-)
对不起，之前为了公平起见，在这帖后，我保持沉默。