Worked solution
Compare a geometric training total with an arithmetic total
(i) Swimmer A swims m on day one and increases each subsequent day’s distance by of the previous day’s distance for days.
(a) Show her day-30 distance is m to the nearest metre. marks
(b) Find her total in 30 days to the nearest metre. marks
(ii) Swimmer B starts at m and adds m each day. Find the least number of days for B’s total to exceed A’s total in (i)(b). marks
[2012/TPJC/Promo/7]
CJC · Review problems · Question 20
Answer
A: m on day 30, total m (nearest metre); B needs days.
(i)(a) Find one term of the geometric progression
, giving m to the nearest metre.
(i)(b) Sum all thirty geometric terms
, giving m to the nearest metre.
(ii) Method 1: solve the arithmetic-total inequality
For B, . Using the source’s rounded target gives , with positive cutoff about . Thus the least positive integer candidate is . Using A’s unrounded total gives the same least integer.
(ii) Method 2: verify adjacent integer totals
The source calculator table is:
| n | B total (metres) |
|---|---|
| 48 | 8040 |
| 49 | 8330 |
| 50 | 8625 |
These totals increase, and , so the least number of days is . The source’s intermediate decimal cutoff is not a fractional permissible day count; round to the next integer satisfying the strict inequality.
Common mistakes
Use a strict comparison and check whole days; premature rounding must not decide the boundary.