Venn and probability questions are some of the most dependable marks in Decision Making — the maths is light and the methods are fixed. The whole skill is being tidy: read the regions carefully, set up the fractions correctly, and don't let a careless slip throw away an easy mark.
Reading a Venn fast
Each region of a Venn means one specific thing. The overlap of two circles is people in both groups; the part of a circle outside any overlap is people in that group only; the area outside all circles is people in none. Before you touch the numbers, point at the region the question is actually asking about — 'only A', 'A and B', 'A but not C' — because most errors come from answering the wrong region.
Watch the 'only' wording
The commonest trap is mixing up 'in A' with 'in A only'. 'In A' includes everyone in the overlaps that touch A; 'in A only' excludes them. When a question gives you a total for a group, decide whether that total already includes the overlaps before you add or subtract — getting this right is usually the whole question.
Constructing a Venn from a stem
When the question describes the groups instead of drawing them, build the diagram yourself and fill it from the inside out. Place the central overlap first, then the pairwise overlaps, then the single-group regions, subtracting as you go so no person is counted twice. If you're given a grand total, the leftover after filling every region is the 'none' group.
Probability: the basics that cover most questions
A probability is the favourable outcomes over the total outcomes — keep that fraction explicit on the noteboard. For 'A or B' (mutually exclusive) you add the probabilities; for 'A and B' (independent events) you multiply them. Reading 'or' as add and 'and' as multiply handles the large majority of DM probability questions.
Use the complement to save work
When a question asks for the probability of 'at least one', it's almost always faster to find the probability of none and subtract from 1. The complement rule — P(event) = 1 minus P(not event) — turns a messy multi-case sum into a single quick calculation, and it's worth reaching for whenever you see 'at least' or 'not'.
Simple data interpretation
Some DM questions hang a chart or short table off a yes/no-does-it-follow set. Treat each statement on its own and check it directly against the data — does the bar really show the greatest change, did that value actually fall, is the average what's claimed? Don't infer trends the chart doesn't support; if the data doesn't confirm it, the answer is No.
Stay neat to keep the marks
Because these questions are winnable, the only thing standing between you and the mark is a careless error. Label your regions, write your fractions out, and double-check whether a total includes overlaps. A few seconds of tidiness here is the cheapest accuracy you'll buy anywhere in the section.