There is a sequence of numbers such that every entry except for the first entry is the arithmetic mean of its two neighboring entries. The 27th entry is 94 and the 94th entry is 39. What's the first entry?
94==F + 26D,
39==F + 93D, solve for F, D
F ==7728/67 - this is the first term
D = - 55/67 - this is common difference.