Solution:

A = B = K =6
C =7
D = H = 1,
E=0 ,
F = I = G = 4,
J=3,
L=5

Now we can rewrite the equations as :
66 = 76 - 10
44 = 60 - 14
16 = 64 - 46
37 = 67 - 30
35 = 53 - 16
10 = 74 - 64

Solution:

199 - 99

Solution:

Kid1, he was left with one rock.

Solution:

40 balls

Solution:

10

Explanation:
Equation 1 + 9 + 11 = 1 can be derived from
One (o) + nine (n) + eleven (e) = one => 1

Similarly for equation,
12 + 11 + 9
Twelve (t) + eleven (e) + nine (n) => ten (10)

Solution:

Colors:
Y B R G
G R B Y
B Y G R
R G Y B


Signs:
x + - /
- / x +
/ - + x
+ x / -

Solution:

1 2 3 4 5 6 and 7 8 6

Since summation of numbers 1-9(1 2 3 4 5 6 7 8 9) is 45 , so its impossible to divide into two equal parts.
Since we cant flip over 6 to make it 9 but we can flip 9 to make it 6 and hence the solution

Solution:

This can be done if you draw a five pointed star and place the balls on the corners and intersection points. See the figure to understand better.

Solution:

To do this, he must be pushing the cork inside the bottle and then taking out the coin.

Solution:

A Spider

Solution:

In the picture you can find the green matchsticks. We have moved them from the bottom and placed them in the middle. It was a bit tricky but now you have your desired result.