The ratio of number of girls to boys in a school of 3600 students is 39 ∶ 41. How many more girls should be admitted to make the ratio 1 ∶ 1?

To solve this problem, we need to follow these steps:

1. Determine the number of boys and girls currently in the school:

   The given ratio is ( 39 : 41 ), and the total number of students is 3600.

   Let ( G ) be the number of girls and ( B ) be the number of boys.

   According to the ratio: \([ \frac{G}{B} = \frac{39}{41} ]\)

   Also, ( G + B = 3600 ).

   Express the numbers of girls and boys in terms of the total number of students:

   Since the ratio is 39:41, we can write: \([ \frac{G}{3600} = \frac{39}{80} \quad \text{and} \quad \frac{B}{3600} = \frac{41}{80} ]\)

   Solving for ( G ) and ( B ):

   \([ G = \left(\frac{39}{80}\right) \times 3600 = 1755 ]\)

    

   \([ B = \left(\frac{41}{80}\right) \times 3600 = 1845 ] \)

   Determine how many more girls need to be admitted to make the ratio 1:1:

   To make the ratio 1:1, the number of girls should equal the number of boys. Currently, there are 1755 girls and 1845 boys.

   We need the number of girls to be 1845 to match the number of boys.

   The number of additional girls required is: [ 1845 - 1755 = 90 ]

So, the correct answer is:

1. 90