a = 17
The fifth number is 27.
By using formula ,
[tex] \sf \: t_{n} = a + (n - 1)d[/tex]
[tex] \sf \therefore t_{5} = 17 + (5 - 1)d[/tex]
[tex] \sf \therefore \:27 = 17 + 4d[/tex]
[tex] \sf \therefore 4d = 27 - 17[/tex]
[tex] \sf \therefore \: 4d = 10[/tex]
[tex] \sf \therefore\: d = \dfrac{10}{4} [/tex]
[tex] \sf \implies \: d = 2.5[/tex]
Therefore,
[tex] \sf \: t_{2} = t_{1} + d = 17 + 2.5 = 19.5[/tex]
[tex] \sf t_{3} = t_{2} + d = 19.5 + 2.5 = 22[/tex]
[tex] \sf t_{4} = t_{3} + d = 22 + 2.5 = 24.5[/tex]
Therefore, the AP is 17, 19.5 , 22, 24.5, 27.........