Opportunity Cost, loan vs. cash

Mortgage rate 7%,  cash saving rate 4%.   Total principle is $X

Opportunity Cost for Loan:  $X * 3.2%  see below.   

$X * 20% * 4% +  $X * 80% * 7% - $X * 80% * 4% = $X ( 20% * 4% + $X * 80% * (3%)) = $X * (0.8% + 2.4%) = $X * 3.2% 

Opportunity cost for Cash:  $X * 4%