In a class of 50 students there are 22 girls who scored an average of 35 marks in the test. What is the average marks of the boys if the class average is 42 marks?

By using alligation method
Total students = 50
Girls = 22 (Given)
Boys = 50 - 22 = 28

\[\begin{array}{*{20}{c}} {}&{28}&:&{22} \\ {{\text{Ratio}} \Rightarrow }&{14}&:&{11} \end{array}\]
14 units = 7
1 unit = $$\frac{7}{{14}}$$
1 unit = $$\frac{1}{2}$$
11 units = 11 × $$\frac{1}{2}$$ = 5.5
Average marks of boys
x = 42 + 5.5 = 47.5