Solution:

The ten difference are as follows:
1) The direction of the paper plane is changed.
2) The amount of white flowers is different.
3) There is one extra bird.
4) The shoes of the girl are plain white.
5) One snail is missing from the picture.
6) There is one extra butterfly.
7) One apple is missing from the picture.
8) Different number of dots of the bow of the girl.
9) There is one extra plant.
10) If you notice, an airplane is missing from the ground.

Solution:

Suzie took the 100 toffees from the largest tin and then she kept one of the smaller tin of toffees inside the empty tin. In this manner, when her sister checked, there were still 100 toffees in each of the tins.

Solution:

When the boats meet for the first time, they have sailed a combined distance that is equal to one length of the lake. When they meet the second time, they have sailed 3 lengths. The elapsed time and the distance for each is three times.

When they meet for the second time, the boat M has sailed 500 x 3 = 1500 yards. Now, this is 300 yards longer than the length of the lake, it must be 1200 yards wide.

The ration between the speed of boat M and boat N is equal to the ratio of the distance that they have sailed before they meet the first time.

Solution:

29 rungs.
Since he is standing on the middle rung in the beginning,
Let the middle rung be x
He climbs 6 rungs first and then climbs down rungs and again climbs 18 rungs to reach the top.
Thus the position is now,
X+6-10+18 = x + 14.
This position is on the top rung.
Thus the ladder has 14 rungs above the middle rung and also it must be having 14 rungs down the middle point as well.
14+14+1 = 29.
Thus there are total 29 rungs in the ladder.

Solution:

If we take the worst case, she will have to take out 12 + 1 = 13 candies.

Solution:

Time

Solution:

10992

Explanation:
Each girl has seven bags with seven adult cats each.
Also, each adult cat has seven little cats.
Thus each girl has 49 adult cats and 343 little cats.

Hence, seven girls will have
343 adult cats and 2410 little cats.

If we calculate the number of legs now.
Girls = 7 * 2 = 14
Cats + Little cats = 2744 * 4 = 10976

Total legs = 10976 + 14 = 10990

Since I also walked in the room, there will be two more legs.
Now total legs = 10990 + 2 = 10992.

Solution:

QHJRPBWF

CYLINDER = UHGQGJWA
Reverse the letters of CYLINDER
REDNILYC
Position increase in first four letters by 3.
REDN becomes UHGQ

Position decrease in last four letters by 2.
ILYC becomes GJWA.

Thus in the same manner
HYDROGEN = QHJRPBWF

Solution:

The first figure tells you where you have to make the cut in the paper. Now you have two pieces of paper. If you rotate the right piece to 180 degrees, you will get what is shown in the figure two. Now the hole is in the center of the paper.

Solution:

Just form an equilateral triangle, planting trees at the vertices. Now find the center of the triangle and plant the fourth three on that point. In this manner you will form a shape of tetrahedron and all the trees will have equal distance between each other.