Prize Puzzle Competition 7

19.I recently found just the things I wanted at a car-boot sale.
The way their prices were displayed was interesting.
FIVE for 10p
TWELVE for 18p
FIFTEEN for 20p
NINETEEN for 24p
I eventually worked out the 'logic' of the prices.
What did ELEVEN cost me?

20.Kim wishes to measure out 4 litres of water, but has only two jugs which, when full, hold 3 litres and 5 litres respectively and have no other markings on them.
Plenty of water is available from a tap.
How can Kim measure out exactly 4 litres?

21.A deltahedron is a 3-dimensional shape all of whose faces are equilateral triangles.
The simplest of these is a regular tetrahedron, which is made of 4 equilateral triangles.
The deltahedron I have has 9 vertices.
How many faces does my deltahedron have?

The winner of Competition #6 was Emma Langford of Furze Platt School, Maidenhead.
The answers were:

16.7 people.
Call them Arthur, Belinda, Charles, Delia, Edward, Fiona, Georgette.
Arthur is the father of Charles.
Belinda is the mother of Delia.
Charles and Delia are married and
Edward, Emma, Georgette are their children.
17.3485 x 2 and 6970 x 1 (for smallest possible)
915 x 64 and 732 x 80 (for largest possible)
18. 90 metres from the wide end
#16 was very popular, though not all the answers given were correct
In many cases it was the only problem attempted.
#17 by contrast produced a very small number of entries, and only a few of those were correct.
#18 attracted a good entry with mostly correct answers.