Remember that unit price how much money per one. For example, if the sale was $12 for 3apples ($12 / 3apples) the the unit price is $4 / apple.
Looking at your problem, 3 kg of wood for $5.55 is the same as $5.55 / 3kg. As a unit price, this is $1.85/kg. Simply divide the numerator by the denominator (5.55 divided by 3).
You can look at this as simplifying fractions.
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So 5 kg of wood for $7.50, or $7.50/5kg, would be $1.5 / kg.
Obviously, the better buy would be the cheaper one: $1.5 / kg.