Initially, Divya and her two friends planned to spend $120 each. With three friends, the total cost was:

$3 \times 120 = 360$

When another friend joins, there are now four friends sharing the total cost. To find out how much each friend would spend after re-dividing the cost equally, we divide the total cost by the number of friends:

$\text{Total cost} = 360$ $\text{Number of friends} = 4$

$\text{Cost per friend} = \frac{360}{4} = 90$

Thus, each friend spent $90 on gas after the re-division.

Would you like any further details or explanations? Here are some related questions to consider:

1. How much money did the fourth friend contribute initially?
2. What would each friend's share have been if there were five friends?
3. How would the cost change if they initially planned to spend $100 each?
4. What is the total amount spent if they add another $40 for snacks?
5. How does the cost change if one friend drops out after re-dividing the costs?

Tip: When sharing costs, it's helpful to calculate the total first and then divide by the number of participants to find individual shares.