Out in the real world you rarely get handed two tidy numbers. You get a ratio and one amount, and you have to work out the rest — how much water for that much cordial, how much yellow for that much blue.
Find One Part, Then Scale
This is the move that cracks every reverse ratio. Say a drink is cordial : water = 1 : 4 and you've poured 60 mL of cordial. Cordial is 1 part, and that part is worth 60 mL. So one part = 60 mL. Water is 4 parts: 4 × 60 = 240 mL. Done. The whole skill is: turn the known amount into "the value of one part", then multiply up for the part you want.
Why It Works
It's the equivalent-ratios idea from rung 1, used in reverse. 1 : 4 scaled up by 60 becomes 60 : 240 — same mix, bigger batch. Finding "one part" is just finding the scale factor that got you from the simple ratio to the real amounts. Once you know it, every other part comes for free.
Sharing in a Ratio
Splitting $50 between two people in the ratio 2 : 3? Add the parts: 2 + 3 = 5 parts in total. One part is 50 ÷ 5 = $10. So one person gets 2 × 10 = $20 and the other 3 × 10 = $30. Same "find one part" move — this time the known amount is the whole, so you divide by the total number of parts.
Why This Is the Finish Line
Recognising a ratio was the start; simplifying made it tidy; dodging the three traps made it safe. But taking a real ratio and a single amount and confidently filling in the rest — that's the bit that shows up in the kitchen, in the shed mixing paint, and in every exam question that starts "share…". That's mastery.