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)