Which ratio will come next in the following series?
1 : 2, 3 : 6, 4 : 8, ?

The given series is: 1 : 2, 3 : 6, 4 : 8, ?

The logic followed is:
All the quantities in the above series maintain the ratio of 1 : 2 with each other.

⇒ 1 : 2
⇒ 3 : 6 = 1 : 2
⇒ 4 : 8 = 1 : 2

Now, check from the options which option maintain the ratio of 1: 2 with each other.

1) 7 : 21 ⇒ 1 : 3

2) 8 : 32 ⇒ 1 : 4

3) 6 : 8 ⇒ 2 : 3

4) 5 : 10 ⇒ 1 : 2

Only option 4) maintains the ratio of 1 : 2 with each other.

Hence, the correct answer is "5 : 10".