What letter comes next in the series: z, x, v, t, ?

A. S

What letter comes next in the series: z, x, v, t, ?

A. S

What is the missing number in the series: 2, 4, 8, 16, ?, 64?

B. 32

Find the missing number in the series: 5, 10, 20, 40, ?

D. 80

What is the missing number in the series: 2, 6, 12, 20, ?, 42

B. 32