Packed lunch for a long walk

Serves three walkers. The walk is eighteen kilometres and you cover four and a half kilometres every hour, so it takes four hours.

Serves $3$ walkers. The walk is $18$ km at $4.5$ km/h: $18÷4.5=4$ h.

$n=3$ walkers. $18÷4.5=4$ h.

Water: each walker drinks about half a litre an hour, so two litres in four hours. Add a quarter more to be safe: two and a half litres each, and seven and a half litres for all three.

Water: $0.5$ L per hour, so $0.5\times 4=2$ L. A quarter extra: $2\times 1.25=2.5$ L each, and $3\times 2.5=7.5$ L for all three.

$w=0.5L/h\times 4\mathrm{h}=2\mathrm{L}$; $2\times 1.25=2.5\mathrm{L}$ and $3\times 2.5=7.5\mathrm{L}$.

Food for each walker: two sandwiches, three dates and one apple. For three walkers that makes six sandwiches, nine dates and three apples.

Each walker: $2$ sandwiches, $3$ dates, $1$ apple. For three: $3\times 2=6$ sandwiches, $3\times 3=9$ dates, $3\times 1=3$ apples.

sandwiches, dates, apples: $3\times (2,3,1)=(6,9,3)$.

Method: pack everything the night before. A litre of water weighs about a kilo, so each pack starts with two and a half kilos of water. After two hours you have drunk a litre and the pack is a kilo lighter. This recipe gets easier as you go.

Method: pack the night before. $1\mathrm{L}\approx 1\mathrm{kg}$ of water, so each pack starts with $2.5$ kg of it. After $2$ h: $2.5-1=1.5$ kg left, a lighter pack.

$1\mathrm{L}\approx 1\mathrm{kg}$; after $2$ h: $2.5-1=1.5\mathrm{kg}$.