A5-Speedy Sums

Let the length of the journey be n km. The first half of the journey takes (1/2×n)/80 = 1/160×n hours,

and the second half of the journey takes 
(1/2×n)/120 = 1/240×n hours. 

The average speed for the complete journey is 
n/(1/160×n+1/240×n) = 96 km/h.

A4- Plus Puzzle

This is how you arrange six blocks

Q5-Speedy Sums

A salesman drives from Amsterdam to The Hague. The first half of the distance of his journey, he drives at a constant speed of 80 km/h. The second half of the distance of his journey, he drives at a constant speed of 120 km/h.

The Question: What is the salesman's average speed for the complete journey?

A Hint : The solution is not 100 km/h!

Q4- Plus Puzzle

The six puzzle pieces shown above can be combined into a symmetrical plus sign.  The Question: How can this be done?