If we know where our consumer would consume on the compensated budget we know exactly how much of the change in quantity demanded was due to the substitution effect.

    When our consumer was on the blue original budget she chose bundle A. This tells us that of all bundles on or below that budget, A was the one she liked the best. In figuring out where on the compensated budget she will consume, we can rule out portions in the shaded area under the blue original budget. That part of the compensated budget that lies under the blue original budget is darkened to remind us that we know she won't pick a bundle on that portion.

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