The fraction of the amount Liang has left is 
- Let X = Amount spent on a pen; X =

- Let Y = Amount spent on books; Y =

- Let M = Amount of money Liang has got; M =?
- Let L = Amount of money Liang will have left; L = ?
<h2>Further Explanation</h2>
To derive L (amount of money Liang will have left), add X (amount spent on a pen) and Y (amount spent on books) together, then the derived value be subtracted from M (fraction of total amount Liang has got) i.e.: M – (X + Y)
Therefore, L = M – (X + Y)
Deriving (X + Y)
= X + Y
=
+ 
= (1 x 7) + (3 x 4) / 21
= (7) + (12) / 21
= 7 + 12 / 21
= 
X + Y = 
Since the lowest common denominator of the fraction of X and Y = 21, then the fraction of M = 
Thus, M = 
Recall, L = M – (X + Y)
Thus,
L =
– 
L = (1 x 21) – (1 x 19) / 21
L = (21) – (19) /21
L = 21 – 19 /21
L = 
Hence, the amount Liang will have left is 
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KEYWORDS:
- denominator
- numerator
- numbers
- fraction
- liang