Final answer:
Thom's monthly phone bill can be determined by multiplying the amount of data used in gigabytes (g) by the cost per gigabyte, which is $15. The equation is B = 15 * g.
Step-by-step explanation:
To write an equation for Thom's monthly phone bill based on the amount of data he uses, we need to incorporate the cost per gigabyte and the number of gigabytes used. Since Thom's plan is $15 per gigabyte of data (represented by 'g'), the equation for calculating Thom's monthly phone bill (B) is straightforward:
B = 15 * g
This equation means that the monthly bill (B) is equal to 15 times the number of gigabytes of data (g) used by Thom. If he doesn't use any data in a month, the bill would be $0. For each gigabyte of data he uses, $15 is added to his bill.