For a statement to be true its contrapositive must also be true .
Let E= Number is even R= Card is red
E –> R
For this even cards must be turned to check if the other side is Red because If E is true then R can’t be false . But we don’t care about the case where E is not true .
Contrapositive of E->R will be R’->E’ ( ‘ is for negation)
For this non red cards must be turned to check if the other side should not be Even because If R’ is true then E’ can’t be false . But we don’t care about the case where R is true .
Truth table for →