Question 294029
Contrapositives are equivalent.
Switch the arguments and negate both
I If there is no food left on the table, then we have had dinner.
.....If we have not  had dinner, then there is food left on the table

II) If we have had dinner, then there is no food left on the table.
III) If there is food left on the table, then we have not had dinner.
II and III are contrapositives and are equivalent.
This was also answered in January
http://www.algebra.com/algebra/homework/word/misc/Miscellaneous_Word_Problems.faq.question.263278.html