if  a card has a D on one side  then  it has a 7 on the other 
if 
P

then 
Q

D

F

7

5

P

not P

Q

not Q

Card #1

Card #2

Card #3

Card #4

Logical answer:
Given the truth table for "if . . . then . . ."
Q is true  Q is false  
P is true  "If P then Q" is true  "If P then Q" is false 
P is false  "If P then Q" is true  "If P then Q" is true 
a proposition of the form If P then Q is falsified if and only if P is true and Q is false.
Since we are looking for falsifying instances  which are cases of P and notQ  we need to check anything that is P (to see if it might also be notQ), and anything that is notQ (to see if it might also be P). Things that are notP and things that are Q are irrelevant.
Therefore, the correct answer, in the Wason trial above, is:
"Card #1 and card #4"  because this corresponds to the instance
of P (card #1) and the instance of notQ (card #4).
(Of course, in real experiments there are many trials, and the order of the cards is varied).
if  someone drinks beer  then  (s)he is 21 or older 
if 
P

then 
Q

beer

diet coke

23 years old

19 years old

P

not P

Q

not Q

Card #1

Card #2

Card #3

Card #4
