From the question stem draw out a double matrix. It is easier to solve with a double matrix rather than forming a equation.
Put two columns B for Boys and G for Girls
Put two rows 7 yrs for 7 yrs students and 8 yrs for 8 yrs students
Now take a nice number so that it is easier to work for Eg. 100 for total students.
Fill up the information:
Boys \((B) =\frac{80}{100} * 100 = 80\)
Hence Girls\((G) = 20\)
Now find the number of 8 yrs old girl \(= 20 * \frac{25}{100} = 5\)
From this find the number of girls who are 7 yrs old = \(20-5 = 15\)
Let x = number of boys that are 7 yrs old
and Y = number of boys that are 8 yrs old
Given,
\(x + 15 = y + 5\)
also we can deduce \(x + y = 80\)
solve the two pair of equation to find the value for x and y
\(x = 35 and Y = 45\)
Therefore, % of boys that are 7 yrs old is 35
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