Just number five a and b . just some !

Dakota earned $7.50 in interest in account a and $18.75 in interest in account b after 15 months. if the simple interest rate is 4% for account a and 5% for account b, which account has the greater principal? explain. to make it a little easier, there is an image. good luck!

Math it's appreciated. (picture provided)

1) prove that 731^3−631^3 is divisible by 100 2) prove that 99^3−74^3 is divisible by 25