hi
not sure of the first line, but I assume you meant
A = ( 3 +3/4) / ( 4+1/2)
then we have : ( 12/4 +3/4) / ( 8/2 +1/2)
15/4 / 9/2
As divising by a number is equal to multiply by it's inverse , then inverse of 9/2 is 2/9 we have :
15/4 * 2/9
As you know, multiplying fraction means multiplty numerator with numerator and denominator with denominator
so : 15/4 * 2/9 = ( 15* 2 ) / ( 4*9) = ( 5*3 *2 ) / (2*2*3*3 ) = 5 / 6
Result is 5/6