I don’t quite agree with the solution. If you check whether using 24 liter of the first mixture (20%) and 40 liters of the second mixture (40%) adds up to a 64 liter mixture with 28% lime juice, it doesn't.
24*0.2 + 40*0.4 = x*64
4.8 + 16 = 64x
20.8 = 64x
x = 20.8/64 = 0.325 => which is not 28%
My solution:
0.2*x + 0.4*(1-x) = 0.28
0.2x + 0.4 - 0.4x = 0.28
0.4 - 0.2x = 0.28
0.12 = 0.2x
x = 0.12/0.2 = 12/20 = 3/5 = 0.6 => the whole mixture consists of 3/5 of mixture 1 and 2/5 of mixture 2.
So x:y = 3:2, therefore the solution is 24 liters of mixture 1 and 16 liters of mixture 2.
24*0.2 + 16*0.4 = x*40
4.8 + 6.4 = 40x
x= 11.2/40 = 0.28 => this way the final mixture will be one of 28% lime juice.