Question 1153031
binomial function n=13 x is number reported and p=0.31 with 1-p=0.69

for 1 exactly it is 13C1*0.31^1*0.69^12, the 13C1=13 ways it happens
this is 0.0197

for at most 1 is the above + probability of 0's occurring or .69^13=0.0080
that probability is 0.0277

Notice how the formula is structured and how the exponents add up to n.