Find the 2nd term of an arithmetic sequence with t1 = 4 and t5 = 6.

Since it's arithmetic we are adding the same number each time.

4,    ,  , ,  6
t1,     t2,        t3,      t4,      t5

Subtract 6-4 = 2
Subtract t5 - t1 = 4
divide 2/4 = 1/2 the common difference is 1/2, we are adding 1/2 each time.
4,  4.5,   5,   5.5,  6
The second term is 4.5