Kept getting the wrong answer? binomial conditional probability

[mtingt]

Product Testing A supposed coffee connoisseur claims she can distinguish between a cup of instant coffee and a cup of drip coffee 75% of the time. You give her 5 cups of coffee and tell her that you will grant her claim if she correctly identifies at least 4 of the 5 cups.

(a) What are her chances of having her claim granted if she is in fact only guessing?

i did
C(5,4)* .75^4 *.25 get the exactly 4 add .75^5 to get exactly 5.
i got 0.6328 but my online homework kept saying that it 's wrong.

 

[bpet]

Hint: consider what would be the probability of success if she was testing a single cup and only guessing.

Edit: this thread belongs in the homework forums