Answer
The initial amount on Jon's giftcard is $48.
Explanation
Let the initial amount on Jon's gift card be x dollars.
Jon pays $17 for a shirt using the gift card and the remaining balance was $31.
That is, the equation representing this is
x - 17 = 31
To solve this, we add 17 to both sides
x - 17 + 17 = 31 + 17
x = 48 dollars
The initial amount on Jon's giftcard is $48.
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