Jumlah deret tak hingga 16+4+1....?

16 + 4 + 1 + ... ?
a = 16 dan r = 4/16=1/4

S∞ = a ÷ (1 - r)
      = 16 ÷ (1 - 1/4)
      = 16 ÷ (4/4 - 1/4)
      = 16 ÷ (3/4)
      = 16 × 4/3
      = 64/3 = 21 1/3