Standardize both deals to the same quantity, then compare. Two methods work:

• Unitary method: find the cost of one unit ($\text{Cost}\div\text{Quantity}$), and compare the unit costs.
• Common multiple method: find the cost of a convenient shared quantity (e.g. per 100 g) for each deal instead — often avoids messy decimals.

Example: 3 caps for £22.50 vs. 5 caps for £36.00.

£7.20 < £7.50, so the second deal is better value.

Convert everything to the same unit first — comparing across units directly is meaningless.

Example: Brand A: 400 g for £2.56. Brand C: 1.2 kg for £7.38. Convert 1.2 kg to $1200$ g. Compare cost per 100 g: Brand A $= £2.56\div4=£0.64$; Brand C $=£7.38\div12=£0.615$. Brand C is cheaper.

• Percentage off the price: reduces the cost (numerator) — calculate the discount and subtract it before dividing.
• Percentage extra free: increases the quantity (denominator) — calculate the bonus amount and add it to the original quantity, but use the original price.

Example: A 120 g bar with "50% extra free" gives a bonus of $60$ g, so you get $180$ g for the original price — a cheaper rate per gram, not per bar.

You pay for fewer items than you receive. Divide the cost of the items you pay for by the total number you actually get.

Example: "3 for 2" at £1.80 each: you pay for 2 items ($£3.60$) but receive 3, so the effective cost per item is $£3.60\div3=£1.20$, not £1.80.

Then the goal shifts from "cheapest per unit" to "cheapest total cost that meets the requirement" — you may have to round the number of packs up, since you can't buy a fraction of a pack.

Example: Need at least 30 batteries. Shop A: packs of 4 at £1.60. Shop B: packs of 6 at £2.70.

Even though Shop A's unit rate (£0.40) is worse than Shop B's isn't the point here — Shop A gives the lower total cost (£12.80), so it's the best buy for this requirement.

A shop sells juice in two sizes: 750 ml for £1.35, or a multi-buy "two 500 ml bottles for £1.80." Which is better value per ml?

1. 750 ml bottle: $£1.35 \div 750 = £0.0018$/ml.
2. Multi-buy: total $1000$ ml for £1.80: $£1.80\div1000=£0.0018$/ml.
3. Both rates are identical (£0.0018/ml, or 0.18p/ml) — there is no best buy; the two options are the same value.