MPM is the amount that imports will increase (or decrease) for every increase (or decrease) in disposable income.
When income increases, those who benefit from it have a choice to either save or spend. If they spend 25% of their increase in income on imported items, their MPM would be 0.25.
Marginal Propensity to Import = Change in Imports / Change in Income
Imports increase by $300 in the same period where income increases by $1,500.
MPM = $300 / $1,500 = 0.2
Therefore, Marginal Propensity to Import is 0.2.