In the first three months, Carson saves $20 in addition to his initial $50, for a total of $70. In the second three months, he saves another $20, for a total of $90. He saves an additional $20 every three months for a total of 12 quarters, or 3 years. Therefore, he saves a total of $20 * 12 = $240 over the course of three years. Adding this to the initial $50 he had, he will have saved a total of $290 for the e-bike.