target sells 24 bottles of water for $3, and 36 bottles of water for $4. Which is better buy and by how much ?

the equation is going to be

answer: $53750

steps:

list the principal and interest after the n-th year:

n       principal   simple interest

1     12500     375

2     25000     750

3     37500     1125

4     50000     1500

sum: $50000     $3750

total at end of year 4: $53750