If the consumption function is C50 0.75y then the marginal propensity to consume is?

average propensity to consume is the fraction of the total amount of disposable income that households spend on consumption whereas marginal propensity to consume is the amount that consumption increases for every additional dollar of disposable income.

The average propensity to consume is the fraction of total disposable income that households spend on consumption (as opposed to saving for example) whereas marginal propensity to consume is the additional consumption that results from an additional dollar of disposable income.

we do care about the marginal propensity to consume because it shows the ratio of an increase in consumption due to increase in income it does not matter what the income of the consumer,either high or low.

1/1-(mpc-mpm)

mpc- marginal propensity to consume

mpm- marginal propensity to import