Miguel saves the same amount of money into a bank account each week. The bank account started with some money. After 3 weeks, the bank account contained $250. After 10 weeks the bank account contained $600. Write an equation to be used as a model the number of dollars, y, Miguel saves in x weeks. Explain the slope and y intercept.

The interest rate would be 2.4%.

We start out finding the total amount he pays, including interest. 

78.40/month * 12 months * 5 years = $4704

We take off the amount of principal:
4704-4200 = 504

This is the amount of interest he pays.  Now we use the simple interest formula, I=prt:

504=4200(r)(5)

Multiply 4200 and 5:
504=21000r

Divide both sides by 21000:
504/21000 = 21000r/21000
0.024 = r
2.4% = r