Mario gave the clerk two 20 dollar bills, which is a total of 40 dollars.
The total cost of the shirt and socks is $28.95 + $4.29 = $33.24.
To find out how much change Mario should receive, we need to subtract the total cost from the amount that he gave to the clerk:
$40.00 - $33.24 = $6.76
Therefore, Mario should receive $6.76 in change.