Top left: A
Top right: B
Bottom right: C
Bottom left: D

A + B = 8 ---- (1)
B + C = 8 ---- (2)
A + D = 13 ---- (3)
D - C = 6 ---- (4)

A pity you have yet to learn how to solve simultaneous equations (taught in Sec 2).

Adding equations 1, 2 and 3 together,

A + B + B + C + A + D = 8 + 8 + 13
A + B + A + B + C + D = 29

But let's not forget in the first place that A + B = 8.

(A + B) + (A + B) + C + D = 29
8 + 8 + C + D = 29
16 + C + D = 29
C + D = 29 - 16
C + D = 13 ---- (5)

Let's group equations 4 and 5 together.

D - C = 6 ---- (4)
D + C = 13 ---- (5)

It's like saying,

1 durian - 1 carrot = 6
1 durian + 1 carrot = 13

Of course, the difference between the above is a quantity of 2 carrots, which is a difference of 7.

So, 2 carrots = 7
1 carrot = 3.5

Then, remember from equation 5 that

1 durian + 1 carrot = 13
1 durian + 3.5 = 13
1 durian = 9.5

So, C = 3.5 and D = 9.5.

But from equation 2,

1 banana + 1 carrot = 8

Since 1 carrot = 3.5,
1 banana + 3.5 = 8
1 banana = 4.5

And from equation 1,
1 apple + 1 banana = 8
1 apple + 4.5 = 8
1 apple = 3.5

So,

A = 3.5 (top left)
B = 4.5 (top right)
C = 3.5 (bottom right)
D = 9.5 (bottom left)

Check that

3.5 + 4.5 = 8
3.5 + 9.5 = 13
4.5 + 3.5 = 8
9.5 - 3.5 = 6

You can use apples, bananas, carrots and durians to represent the quantities in each box and then do from there.