The maths of ratios is easy. The marks are lost on three small habits — so let's name them before they bite.
Trap 1 — Order Matters
A ratio reads left to right, and the first number always names the first thing. So cordial : water = 2 : 3 is a different drink from 3 : 2 — one is weaker, one is stronger. Unlike adding (where 2 + 3 = 3 + 2), a ratio is not the same when you swap the numbers. Always match each number to its label, in order.
Trap 2 — Same Units First
You can only compare like with like. If a question gives 50 cm : 2 m, writing "50 : 2" is wrong — you'd be comparing centimetres against metres. Convert one first: 2 m = 200 cm, so it's really 50 : 200 = 1 : 4. Get both parts into the same unit before you simplify.
Trap 3 — It's Not a Fraction of the Whole
If a class is girls : boys = 2 : 3, it's tempting to say "girls are 2/3 of the class". They're not. Add the parts: 2 + 3 = 5. So there are 5 parts in total, and girls are 2 out of 5 (that's 2/5), boys 3 out of 5. A ratio compares the two groups to each other; to turn it into a fraction of the whole, you have to add the parts first to find the total.