Asimov, in his chapter on Seth Low (ch 14) doesn't give any dates, so just assume it's sometime during the latter half of 1935, whenever school began in New York back then.
Asimov very rarely mentions his sister in his biography (I think the only time he does so is when she gets married) and mentions his brother, some 8 years or so his junior, only slightly more frequently. He tells the story of how it was he who took his brother to his first day of school - when he was old enough to go, and how Stanley got along so much easier, not only with his peers at school but also with his parents.
Asimov went to City College for 3 days - took a physical exam for which he was labeled PD - poorly developed - and an IQ test, which apparently astonished his profssors, and they wanted to test him so more, when he got a letter from Seth Low College, which his father opened.
The letter asked Asimov why he had failed to show up for registration. His father called the school immediately and explained they didn't have the money for tuition. He was immediately offered a $100 scholarship for his freshman year, "to be repaid at our leisure", and a job with the National Youth Administration (NYA) that would net him a further $15 a month.
So Asimov went to Seth Low, which was located in the Boro Hall section of Brooklyn, about 4 and a half miles due west of the Decatur Street candy store. He took the subway there and back. It cost 5 cents, one way.
According to Asimov, writing in 1979, "In those days, of course, subways were clean and safe, charged five cents a trip, and were terribly crowded. If you want dirt and danger you have to pay for it, so nowadays the fare is fifty cents and no one takes the subway who can avoid it."
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