First, we figure out the price per meal by adding the steak and the drink together, finding a cost of 20+5=<<20+5=25>>25 per meal.
Then we multiply this by 2, as there are two people eating, meaning a total cost of 25*2=<<25*2=50>>50 for both meals.
Then we find the total tip amount for a 20% tip, by multiplying 50*.2= 10.
Then we find out what 80% of this tip amount is by multiplying the desired tip of 10 by .8, finding 10*.8= <<10*.8=8>>8 dollars that Billy should contribute.
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