To determine how much Mary will save in 15 weeks, we first need to find her weekly savings rate.

Mary saved $17.50 in 3 weeks. Therefore, her weekly savings rate is: $\frac{17.50}{3} = 5.83 \, (\text{rounded to 2 decimal places})$ So, Mary saves approximately $5.83 per week.

Now, to find out how much she will save in 15 weeks, we multiply her weekly savings by 15: $5.83 \times 15 = 87.45$

Thus, Mary will save $87.45 in 15 weeks.

Would you like more details or have any questions about this? Here are 5 related questions for practice:

1. If Mary saved $20 in 4 weeks, what would her savings be in 12 weeks?
2. If Mary increased her savings by $1 per week, how much would she save in 15 weeks?
3. What is the total amount saved if Mary saved $5 every week for 20 weeks?
4. If Mary saved $30 in 6 weeks, what is her weekly savings rate?
5. How much will Mary save in 10 weeks if her savings rate is $6.25 per week?

Tip: For questions involving consistent rates, multiplying the rate by the number of periods is key to finding the total.