Using the De Morgan Rule

!(A || B) is equivalent to !A && !B

The expression

boolean oilOK =  !( months >= 3 || miles >= 3000 );

is equivalent to

boolean oilOK =  !( months >= 3 ) && !( miles >= 3000 );

which can be further transformed to

boolean oilOK =  ( months <  3 ) &&  ( miles <  3000 );

Free Shipping

For an on-line shopping site, shipping is free for purchases of $50 or more, unless the merchandise was on sale.

boolean freeShipping =  purchase >= 50 && !onSale;

Here is an expression that shows when to charge for shipping:

boolean shipping =  !( purchase >= 50 && !onSale );

Assume that onSale is a boolean variable.

Rewrite this expression in two steps. In the first step, apply De Morgan's rule that !(A&&B) is equivalent to !(A)||!(B).

boolean shipping =

In the second step, rewrite the relational expression:

boolean shipping =