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This one took a lot of patience - well done!

This well explained solution comes from Gemma who goes to Foxford School and Community College, Coventry.

Remember: the jugs hold $9$, $7$, $4$ and $2$ litres, and you can't guess any amounts.

9 0 0 0
From the $9$ litre bottle take $4$ litres to $4$ litre bottle.
5 0 4 0
$4$ litres from $4$ litre bottle to $7$ litre bottle.
5 4 0 0
Take $4$ litres from the $9$ litre bottle to the $4$ litre bottle.
1 4 4 0
$3$ litres from the $4$ litre bottle to $7$ litre bottle.
1 7 1 0
$2$ litres from $7$ litre bottle to $2$ litre bottle.
1 5 1 2
$2$ litres from the $2$ litre bottle to $9$ litre bottle.
3 5 1 0
$2$ litres from $7$ litre bottle to $2$ litre bottle.
3 3 1 2
$2$ litres from $2$ litre bottle into $4$ litre bottle.
3 3 3 0

Syed also from Foxford School and Community College, Coventry has found a different solution, and presented it in a table.

Amount of drink in the jug (litres) The Pour
$9$ - litre jug $7$ - litre jug $4$ - litre jug $2$ - litre jug
Full Empty Empty Empty Start
$2$ litres Full Empty Empty $9$ - litre jug to $7$ - litre jug
Empty Full $2$ litres Empty $9$ - litre jug to $4$ - litre jug
Empty $5$ litres $2$ litres Full $7$ - litre jug to $2$ - litre jug
Empty $3$ litres Full Full $7$ - litre jug to $4$ - litre jug
$4$ litres $3$ litres Empty Full $4$ - litre jug to $9$ - litre jug
$6$ litres $3$ litres Empty Empty $2$ - litre jug to $9$ - litre jug
$6$ litres $1$ litre Empty Full $7$ - litre jug to $2$ - litre jug
$6$ litres Empty $1$ litre Full $7$ - litre jug to $4$ - litre jug
$6$ litres $2$ litres $1$ litre Empty $2$ - litre jug to $7$ - litre jug
$1$ litre Full $1$ litre Empty $9$ - litre jug to $7$ - litre jug
$1$ litre $5$ litres $1$ litre Full $7$ - litre jug to $2$ - litre jug
$3$ litres $5$ litres $1$ litre Empty $2$ - litre jug to $9$ - litre jug
$3$ litres $3$ litres $1$ litre Full $7$ - litre jug to $2$ - litre jug
$3$ litres $3$ litres $3$ litres Empty $2$ - litre jug to $4$ - litre jug

Here's a third solution from Thomas at Tattingstone Primary, UK.

9 0 0 0
 
2 7 0 0
 
2 5 0 2
 
4 5 0 0
 
4 3 0 2
 
6 3 0 0
 
6 1 0 2
 
8 1 0 0
 
8 0 1 0
 
1 7 1 0
 
1 5 1 2
 
3 5 1 0
 
3 3 1 2
 
3 3 3 0