The amount of money that Davis has in this scenario is; $50
Let the amount of money he has be $X.
Now, if he buys 10 notebooks, he will have $10 left. Thus;
X - 10n = 10 -----(1)
If he buys 15 notebooks, he will be $10 short. Thus;
X - 15n = -10 ------(2)
Subtract eq 1 from eq 2 to get;
-5n = -20
n = -20/-5
n = 4
Thus;
X - 10(4) = 10
X = 10 + 40
n = $50
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