Let the number of months be x.
For Dealer 1:
After x months, amount paid = 150*x = 150x.
Total = 2500 + 150x
For Dealer 2:
After x months, amount paid = 125*x = 125x.
Total = 3000 + 125x
When they will be equal:
2500 + 150x = 3000 + 125x
150x - 125x = 3000 - 2500
25x = 500
x = 500 / 25
x = 20.
So after 20 months.
Amount = 2500 + 150x, x = 20
2500 + 150*20 = 5500
So after 20 months the amount for both dealers would be the same. And the amount the same is $5500.