Good X sells for price PX. Since our consumer has income I, the amount of X she can purchase if she spends all her money on X is equal to I ÷ PX. This gives us the location of the endpoint along the X, or horizontal, axis.

    In the same way we can compute the endpoint of the budget constraint along the Y, or vertical, axis. This turns out to be simply I ÷ PY.

    Finally, the slope is just the negative of the price ratio, or -PX ÷ PY.

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