The slope of any isoquant is the MRTS which turns out to the the ratio of the Marginal Product of Labor to the Marginal Product of Capital. Remember that the MPL is the added output due to a small increase in labor while the MPK is the added output due to a small increase in capital. The ratio of these two will tell us at what rate capital and labor may be substituted for one another while maintaining output.

    Suppose - MPL/MPK = 2. This would mean that if labor were reduced by one worker capital would need to be increased by 2 units to maintain output (remain on the isoquant). Since the isoquant is a curve the MRTS is different at each point.

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