Given a list of values and a corresponding list of probabilities, the Excel PROB function calculates the probability associated with a given range.
The format of the Excel Prob function is:
where the arguments are as follows :
x_range    An array of values, which have associated probabilities 
prob_range   
The array of probabilities corresponding to the values in x_range
(This array must have the same length as the x_range array and the values in prob_range must add up to 1) 
lower_limit    The lower bound of the range of values for which you want to calculate the probability 
[upper_limit]   
An optional value that denotes the upper bound of the range that you want to calculate the probability for.
(If the upper_bound is omitted the function simply returns the probability associated with the value of the supplied lower_bound) 
Columns A and B of the spreadsheet below contain a set of values and their associated probabilities. These values and probabilities are provided to the Excel Prob function, in cells C1C3 of the spreadsheet.
Further examples of the Excel Prob function can be found on the Microsoft Office website.
If you get an error returned from the Excel Prob Function, this is likely to be one of the following:
#NUM!   
Occurs if either:

#N/A!    Occurs if the supplied x_range and prob_range arrays are of different lengths (i.e. contain different numbers of data points) 