American vs. Euro Style option

CFA Level I Derivatives
#1

I understand that Americans should be worth at least as much as Euro style options (as American option could be exercised anytime it is above the strike price).

But, what is the best way to approach a question like below (from a theoretical and mathematical standpoint) ?

Q) What is the min values of American and a Euro style 3 month call option wih a strike price of $80 on a non-dividend paying stock trading at $86 if the risk free rate is 3%?

A) American & Euro - $6.59

#2

Use put call parity for european option:

Call = Stock - X/(1+r) + Put; where X=exercise price, r=risk free rate = 3%/4 = 0.75% for 3 months

Since value of Put >=0

Call >= Stock - X/(1+r) = 86 - 80/(1.0075)=6.59