Answer:
210
Explanation:
Find the least common multiple.
Start at 0 with both numbers, and list each multiple until they match up.
In this case:
30: 0, 30, 60, 90, 120, 150, 180, 210, 240...
35: 0, 35, 70, 105, 140, 175, 210...
LCM = 210, The 210th customer will be the first to win both prizes.