Probability of getting at least 3 means the probability of getting 3 or the probability of getting 4 correct. 

Use the BINOMIAL PROBABILITY formula: 

Probability of getting 3 correct:  

Probability of getting 4 correct:  

Probability of getting at least 3 correct (probability of getting 3 or 4 correct): 

OR

Use the BINOMCDF feature on a calculator (e.g. TI-83/84). Enter: binomcdf(4,.5,1). You should also get 

OR

Use MS Excel. Enter: "=1-binomdist(3-1,4,.5,1." You should also get the above answer. 

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