Hi, to answer this question we have to analyze the information given:
Bill for additional data usage: $28.50
Cost per gigabyte: $9
To calculate the additional gigabytes we have to write an expression where the total bill ($28.5) is equal to the gigabytes used (g) multiplied by the cost per gigabyte.
Mathematically speaking:
28.5 = 9g
The three first options are correct (one is put in a different order but is the same expression). So, the incorrect option is (28.59=g).
