You have two methods to solve this problem 1) Algebric - Working with formulas to arrive to a fraction of new students /total students 2) Plug in smart numbers
I would suggest you to plug in smart numbers so let S0 be student before enlarging capacity and be equal to 1000 and let C0 be the original capacity and be 1000 (remember that in origin the location was full up to its capacity thus students=maximum capacity of studio in terms of student).
Then be S1 (students after reduction of 20%)= 1000* (80/100) = 800
Then C1 (capacity after 45% increase) = 1000 * (145/100) =1450
Given that the new studio will be full again we have to add a X number of new students so that the ratio of students to new capacity equals zero
Thus (800/1450) + (X/1450) =1 and so we need 650 new students.
650 new students to a total of 1450 students is 13/29 thus D