Finding missing values in a relative frequency table

Question

A table of some of the relative frequencies computed from a data set is 

 

 

x|    1   2   3   4

————————

f/n| .3  p   .2  .1

 

 

 

the number p is yet to be computed. Find p.

Answer 1

I think p=0.4 so that the sum of all probabilities will be 1. (asumming that the only possible x values are the 1 through 4, as listed.)