To find out how many gallons of gas Becky's car will use on the 2,100-mile trip, we need to divide the total miles by the car's miles per gallon (mpg).

The formula is:

$\text{Gallons used} = \frac{\text{Total miles}}{\text{Miles per gallon}}$

Substituting the values:

$\text{Gallons used} = \frac{2,100 \text{ miles}}{30 \text{ mpg}} = 70 \text{ gallons}$

So, Becky’s car will use 70 gallons of gas on the trip.

Would you like any more details or clarifications?

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Here are 5 follow-up questions to expand on this:

1. What is the cost of gas if the price per gallon is $3.50?
2. How long would the trip take if Becky is driving at an average speed of 60 miles per hour?
3. How much will the trip cost if her car's fuel efficiency was 25 miles per gallon?
4. If Becky wants to reduce the number of gallons used by 20%, how many gallons will she save?
5. How far could Becky travel if she had 80 gallons of gas?

Tip: Always calculate fuel efficiency in miles per gallon (mpg) when planning long trips to estimate how much fuel you'll need.