Answer:
3
Explanation:
As with any other addition fact problem, you can isolate the unknown term by subtracting the unwanted constant.
5/7 +x/6 = 51/42
x/6 = 51/42 -5/7 . . . . . subtract 5/7 from both sides
x/6 = 51/42 -30/42 = 21/42 = 1/2 . . . . . complete the subtraction
x = 6(1/2) = 3 . . . . . . . . multiply by 6
The missing numerator is 3.
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Alternate solution
Multiply the equation by 42:
30 +7x = 51
7x = 51 -30 = 21 . . . . . subtract 30
x = 21/7 = 3 . . . . . . . . divide by the coefficient of x