For numbers 10, 20, 30, 40, 50, 60, 70 and 80; the inter-quartile range is :

1. 20 + 60

2. 60 - 30

3. 60 - 20

4. 50 - 10

Numbers in ascending order : 10, 20, 30, 40, 50, 60, 70 and 80.

The number of terms (n) are 8, which is even.

∴ Lower quartile = $\dfrac{n}{4} = \dfrac{8}{4}$ = 2^nd term = 20.

∴ Upper quartile = $\dfrac{3n}{4} = \dfrac{3 \times 8}{4} = 3 \times 2$ = 6^th term = 60.

Inter-quartile = Upper quartile - Lower quartile = 60 - 20.

Hence, Option 3 is the correct option.