My arithmetic sequence : 4, 8, 12, 16, 20 (determine t_{50})

t_{50} = t_1 + ( n – 1 )( d )

t_{50} = 4 + ( 50 – 1 ) ( 4 )

t_{50} = 4 + 196

t_{50} = 200

Finding the sum of 50 terms : 

S_{50} = \frac{n}{2} (t_1 + t_n )

S_{50} = \frac{50}{2} (4 + 200 )

S_{50} = 25 ( 204 )

S_{50} = 5100