The average score of 42 students in a test is 69. The ratio of the number of boys to that of girls is 10 : 11. The average score of the boys is 20% more than that of the girls. The average score of the boys is:

Total score = $$69\times 42 $$ = 2898

Ratio Boys : Girls = $$10x : 11x $$   (where $$ x$$ is propostional constant)

total student = $$ 10x + 11x = 21 x $$ 

According to question  now girls score is 1 and boys score is 1.2 

then total student score = $$10\times 1.2x + 11\times1  x  =23 x $$  

so $$ x = \dfrac {2858}{23}$$ 

$$\Rightarrow x = 126 $$

Boys score Average = $$ \dfrac{126\times 12}{20}$$ 

Boys score Average = $$75.6$$ Ans