The root to this fraction ratio is 4:5.

By trial and error and knowing the factors to each multiples, work with the multiplication:

4*4 = 16 : 5*4 = 20
4*9 = 36 : 5*9 = 45

Alternatively, use algebraic expressions to manipulate the homogeneous equation:

Let the unknown be X;

16/X = 36/45

Multiple both sides by X and 45,

16*X*45/X = 36*X*45/45

Cancel out common numerator and dominator.

16*45 = 80 + 240 + 400 = 720 = 36X

720/36 = X

20 = X

Hence, 16:20 is equal to 36:45 (sure way of getting the concept)

im Curious if a primary 5 have learned to express unknowns in the form of algebraic expressions or do they still perform methods of trial and error as I have use in method one to solve it. Been 10 years since my syllabus.