Answer:
106°
Step-by-step explanation:
Supplementary angles add up to 180°
180°- 74° 
= 106°
 
        
                    
             
        
        
        
So if 6 out of 40 are incorrect, 40-6= 36 must be correct.
In order to gain percentage you do proportions. 
36/40 = x/100. 
Multiply 36 times 100 then divide by 40, which is 90. 
The answer is 90%.
90% of the questions he got were correct.