Answer:
Option B
The measure of angle b is 75°
Explanation:
Method 1
we know that
In a inscribed quadrilateral, the opposite angles are supplementary
so
∠a+60°=180° ------> equation A
∠b+105°=180° -----> equation B
To find the measure of angle b solve the equation B
∠b+105°=180°
Subtract 105° both sides
∠b+105°-105°=180°-105°
∠b=75°
Method 2
see the attached figure with letters to better understand the problem
we know that
The inscribed angle measures half that of the arc comprising
so
∠105°=(1/2)[arc ADC]
arc ADC=2*105°=210°
Find the measure of arc ABC
we know that
arc ABC+arc ADC=360° -----> by complete circle
arc ABC=360°-210°=150°
Find the measure of inscribed angle b
∠b=(1/2)[arc ABC]
substitute
∠b=(1/2)[arc 150°]=75°