Ratios: Two questions & two different methods of producing the answer. Why?

#1. Each marble bag sold by Eric's Marble Company contains 3 orange marbles for every 5 red marbles. If a bag has 45 red marbles, how many orange marbles does it contain?

answer method: 3 orange/5 red = x / 45 total

#2: Petrolyn motor oil is a combination of natural oil and synthetic oil. It contains 8 liters of natural oil for every 5 liters of synthetic oil. If 415 liters of
synthetic oil are used, how many liters of Petrolyn oil will be made?

5 L synthetic / 13 (5L+8L total liters) = 415 Synth/ x"

Question

Ratios: Two questions & two different methods of producing the answer. Why?

#1. Each marble bag sold by Eric's Marble Company contains 3 orange marbles for every 5 red marbles. If a bag has 45 red marbles, how many orange marbles does it contain?  

answer method: 3 orange/5 red = x / 45 total

#2: Petrolyn motor oil is a combination of natural oil and synthetic oil. It contains 8 liters of natural oil for every 5 liters of synthetic oil. If 415 liters of 
synthetic oil are used, how many liters of Petrolyn oil will be made?  

5 L synthetic / 13 (5L+8L total liters) = 415 Synth/ x"