very basic question- t statistic

Hello,

I have left the school a long long period ago. the question could be very simple and i hope someone could explain it easily for me.

I have 28 sample and from each sample I computed a ratio (example how many x observation in each sample). And finally I took the average of the 28 samples. and I obtained the mean. My question is how to calculate the t statistic for this mean? (are the degrees of freedom 28-1=27? or the sum of obseravtions in all samples-1?. )
thx
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