Question 502465

 number of 4 - letter strings that don't contain G = 24^4

 number of 4 - letter strings that don't contain T = 24^4

 number of 4- letter strings that don't contain neither G nor T = 24^4 
    

 number of four-letter strings of the alphabet, that contain at least one "G" AND

 at least one "T." = 26^4 - ( 25^4 + 25^4 - 24^4)

  

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