1/2+1/4+1/8 what is this

In two or more complete sentences describe how to determine the appropriate model for the set of data, (1,1), (3,2), (6,3), (11,4).

Cassie is traveling 675 miles today to visit her brother and his family. so far, she has stopped for gas three times. at the first stop, she put in 7.1 gallons of gas and paid $23.79. at her second stop, she put in 5.8 gallons of gas and paid $19.08. and, at the third stop, 6.6 gallons and paid $20.39. at which stop did casey pay the most for gas?

2.5 milligrams is equivalent to how many grams