Four angles of a quadrilateral are in arithmetic progression with a common difference of 20 degrees. What is the smallest angle of that quadrilateral?

Correct option is B

Given:
There are 4 angles in arithmetic progression.
Common difference = 20 degrees
Sum of all interior angles of a quadrilateral = 360 degrees

Formula:
For 4 terms in A.P.:
​$$\text{Sum} = \frac{n}{2} \left[ 2a + (n - 1)d \right]$$​​
Where:
n = 4, d = 20, a = first (smallest) angle

Solution:

360 = 4/2 × [2a + 3d]
=> 360 = 2 × [2a + 60]
=> 360 = 4a + 120

4a + 120 = 360
=> 4a = 240
=> a = 60

Final Answer:
(B) 60 degrees