Traps I've hit (read before the exam)
Before each answer, check the trap list for that question type.
- Quartiles: leave the median OUT of both halves when n is odd. Sort before anything.
- 'Given A' → denominator = A's total only. Overlap (queen AND red) → subtract it once.
- 'Between 33 and 35' includes both ends. 'At least' → cdf k to n, or 1 − P(fewer).
- C(n,0) = 1, x⁰ = 1, 0! = 1. Multiply percents as decimals. Reduce every fraction.
- Select / choose with no roles → combination. Titles, order, codes → permutation.
Traps
- Quartiles with the median inside the halves.n odd → median kept in both halves✓ leave the median OUT; sort before anything
- 'Given A' with the grand total.÷ everyone✓ denominator = A's total only
- Overlap counted twice.queen or red → 30/52✓ subtract it once → 28/52 = 7/13
- Wrong binomialcdf bounds.at least k → lower k + 1✓ at least → cdf k to n, or 1 − P(fewer)
- Percents multiplied as whole numbers.(3%)(5%) = 15%✓ multiply as decimals: 0.15%
- Unreduced fraction.15/216✓ 5/72