Final answer:
525 people can be seated on 15 tandem bicycles.
Step-by-step explanation:
In order to determine how many people can be seated on 15 tandem bicycles, we need to multiply the capacity of one tandem bicycle by 15. Given that the longest tandem bicycle can accommodate 35 people, we have:
Number of people on 15 tandem bicycles = Capacity of one tandem bicycle x Number of tandem bicycles
Number of people on 15 tandem bicycles = 35 x 15 = 525
Therefore, 525 people can be seated on 15 tandem bicycles.