This is a multi-step Fractions and Ratios question — the kind that looks complex but becomes very clean once you pick a smart starting number. The key concept is Smart Number / Picking Values with fractions.
The common trap: students try to track percentages and fractions algebraically, lose track of which group is which, and end up with the wrong denominator. Picking 60 as your starting total makes everything work out to whole numbers.
Step 1 — Assign a starting number: Let the current total = 60 houses.
• Currently WA: (1/4) × 60 = 15
• Currently not WA: 45
Step 2 — Work out what happens by 2040:
• Total houses increases by 50%: new total = 60 × 1.5 = 90
• One-third of non-WA houses are demolished: (1/3) × 45 = 15 demolished
• Remaining old non-WA (not demolished): 45 − 15 = 30
• Old WA houses remain: 15 (no WA houses are demolished)
• Old houses remaining: 15 + 30 = 45
• New houses built: 90 − 45 = 45 new houses
• New houses that are WA: (3/5) × 45 = 27
Step 3 — Calculate total WA houses in 2040 before conversions:
WA so far = 15 (old WA) + 27 (new WA) = 42
Step 4 — Find the required WA count and the shortfall:
Required: at least (1/2) × 90 = 45 WA houses
Shortfall = 45 − 42 = 3 houses need to be converted
Step 5 — Find the fraction of eligible houses that must be converted:
Eligible houses = old non-WA houses that were NOT demolished = 30
Fraction = 3/30 = 1/10
The answer is A.
Common trap: Some students use the total new houses (45) as the denominator instead of the eligible old non-WA survivors (30). The question specifically asks about "houses that are currently not WA AND will not be demolished" — those are only the 30 old surviving non-WA houses.
Takeaway: On multi-condition fraction problems, always list out each subgroup separately before doing any arithmetic — rushing to combine groups is where most errors happen.