Question

If the 16th term of an arithmetic sequence is three times the fourth term, find the ratio of the 23rd term to the third term.
translate from English to Math

"If the 16th term of an arithmetic sequence is three times the fourth term"
--->a+15d = 3(a+3d)
a+15d = 3a+3d
12d = 2a
a = 6d

term 23 = a+22d = 6d+22d = 28d
term 3 = a + 2d = 6d + 2d = 8d

term23 : term3 = 28d : 8d
= 28 : 8
= 7 : 2
isn't it supposed to be 3a + 9d?

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