Magic square

$24+x+z=25+z+21$

$24+x=25+21$

$x=22$

$25+22+w=21+y+w$

$25+22=21+y$

$y=26$

Sum of each row is $18+22+26=66$

$25+21+z=66$

$z=20$

$\therefore y+z=26+20=\boxed{46}$

$v+24+w=v+18+25$ $\implies$ $w=43-24=19$

$z+25+21=24+x+z$ $\implies$ $x=46-24=22$

$w+y+21=w+x+25$ $\implies$ $y=22+25-21=26$

$25+z+21=w+y+21$ $\implies$ $46+z=19+26+21$ $\implies$ $z=66-46=20$

Finally,

$y+z=26+20=$ $\boxed{46}$

The square is

Sum is 66. $y+z=20+26=\boxed{46}$

W+Y+21=V+18+25, V+X+21=W+X+25 V-W=4 ,put this in 1st equation we get Y=26 18+X+Y=25+Z+21 W+Y+21=W+X+25 Put Y=26 in above ,we get X=22 Then put X=22,Y=26 in 3rd equation we get Z=20 Hence Y+Z=46

18 + 25 + v = magic number

18 + 25 = 43

43 + v = magic number

24 + w + v = magic number

24 + w + v the row has v:

24 + w = 43

w = 19

same method with diagonal:

21 + x + v = magic number

21 + x = 43

18 + 25 + v = magic number

43 + v = magic number

21 + x + v has v

Anyway...

21 + x = 43

x = 22

Now we have the other diagonal complete

25 + 22 + 19

25 + 22 + 19 = magic number

25 + 22 + 19 = 66

66 = magic number

Now calculate z and y

22 + 24 + z = 66

z = 20

19 + 21 + y = 66

y = 26

20 + 26 = z + y = 46, the correct answer.

Did anyone try it using systems of equations?

I simplified the process and noticed this:

24+ w+ v (eliminated)

25+18+ v (eliminated)

1 + 18 = 19 for w

21 + v + x (eliminated)

25+ 19 + x (eliminated)

4 + 19 = 23 for v

18 + x+ y (eliminated)

21+19+y (eliminated)

3 +19 = 22 for x

Thanks.