Step-by-step answer:
Difference     21%                     35%
     |===================|=============================|
    23%                44%                          79%
The ratio of 23% alloy to 79% alloy is given as
23% : 79% = 35 : 21 = 5:3
(see above diagram)
So weight of each alloy is given by:
23% alloy = 5/(5+3) * 48 kg = 30 kg
79% alloy = 3/(5+3) * 48 kg = 18 kg
Check:
total weight = 30+18 kg = 48 kg
Total weight of copper in 48 kg of 44% alloy = 48*0.44 = 21.12 kg
Total weight of copper of mixed alloys= 30*0.23 + 18*0.79 = 21.12 kg (checks)
