Let t2 be thought of as t1, so that t5 becomes t4, and t10 becomes t9.

Use tn = t1 + (n - 1)d, letting t4 = 16,n = 9, and t1 = 2.  Solve for d.

Then substitute the value of d into the same formula, using n = 9 and t1 = 2.

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