For each person the probability of he/she having a bloodtype O negative is 7% then the probability of one person having a different blood type is 93%. We want all of them to have a different bloodtype, therefore we need to calculate the product of each to have a different bloodtype.
[tex]p=(\frac{93}{100})^4=0.93^4=0.748[/tex]The probability is 0.748 or 74.8%.
