100th customer is your answer
Find the first number that shows up on both answers when they are added
Add 20 each time for $5 gift card:
20, 40, 60, 80, 100
Add 50 each time for $10 gift card:
50, 100
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As seen above, the first number that shows up on both is 100, therefore, 100 is your answer
hope this helps
