Part A
Your problem statement already shows you the conversion.
Bottles purchased: 5×10¹⁰
Bottles recycled: 8.3×10⁹
Part B
The ratio of bottles purchased to bottles recycled is
... (5×10¹⁰)/(8.3×10⁹) = 0.6×10¹ = 6.0
The number of bottles purchased is 6 times the number of bottles recycle.