First reduce it.
10:7 In a sense that is about as far down as you can go
You could however make it 1 3/7 to 1
Have you been taught the butterfly technique
The standard deviation of the sample proportion is:

The z-score for 0.7 is:

Reference to a standard normal distribution table shows that the probability that at least 70% of the sample prefers coke to pepsi is 0.8944 or 89.44%.
You should look for outliers. If there is an outlier then you should use median because if you use mean the data would be skewed.