To find out how much Mom paid for the t-shirts, you can simply multiply the number of t-shirts, $T$, by the cost of each t-shirt, $d$:

$\text{Total cost} = T \times d$

So, if Mom bought $T$ t-shirts and each cost $d$ dollars, the total she paid is $T \times d$ dollars.

Would you like a more detailed explanation or example?

Further questions:

1. What happens if the cost of each t-shirt increases or decreases?
2. How would the total change if Mom bought more t-shirts?
3. What would the formula be if she bought t-shirts at different prices?
4. How can we calculate the cost if there’s a discount on the total price?
5. If Mom uses a coupon that gives a discount on the total cost, how would you adjust the formula?

Tip: Always check the units when multiplying, e.g., dollars for cost and quantity for t-shirts.