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You need the skill of determining the LCD of each pair of fractions and that of converting each fraction so that each has the LCD as denominator.

40 6/7 - 12 1/3 = ?

1.  Determine the LCD.  It's (7)(3) = 21.                                                     3(6)
2.  Convert 6/7 to the equivalent form that has denominatory 21:  6/7 = ----------
                                                                                                                    3(7)

     and this is equal to 18/21.

      We are subtracting 12 7/21 from 40 18/21

3   Now do the actual subtraction:

      40 18/21
     -12 7/21
  ----------------
       28 11/21 (answer)


Next problem:     Evaluate   18 - 10 3/15

Subtracting 10 from 18 is easy:  It's 8.

But subtracting 10 3/15 from 18 requires "borrowing" from the 18:

18 = 17 15/15.

Now you are to subtract:          17 15/15
                                               -  10 3/15
                                             -----------------
                                                    7 12/15           (answer) 

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