The answer is B. You need to find a common denominator. B) 1/4 - 1/5 = 1/20. To get a common denominator, multiply 5 and 4 to get 20. The common denominator is now 20. Now you have to multiply the numerators by the opposite denominator. This gives you 5/20 - 4/20 which equals 1/20 because you are subtracting the numerators, 5 and 4.