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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