Ruben is driving along a highway that passes through Barstow. His distance d, in miles, from Barstow is given by the equation d = |210 − 60t|,
where t is the time, in hours, since the start of his trip and
0 ≤ t ≤ 5.
When will Ruben be exactly 90 miles from Barstow? (Enter your answers as a comma-separated list.)