Porisma

Worked solution

Compare a geometric training total with an arithmetic total

(i) Swimmer A swims 5050 m on day one and increases each subsequent day’s distance by 10%10\% of the previous day’s distance for 3030 days.

(a) Show her day-30 distance is 793793 m to the nearest metre. marks

(b) Find her total in 30 days to the nearest metre. marks

(ii) Swimmer B starts at 5050 m and adds 55 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: 793793 m on day 30, total 82258225 m (nearest metre); B needs 4949 days.

(i)(a) Find one term of the geometric progression

u30=50(1.1)29≈793.15465u_{30}=50(1.1)^{29}\approx793.15465, giving 793793 m to the nearest metre.

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(i)(b) Sum all thirty geometric terms

T=50[(1.1)30−1]/(1.1−1)≈8224.70113T=50[(1.1)^{30}-1]/(1.1-1)\approx8224.70113, giving 82258225 m to the nearest metre.

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(ii) Method 1: solve the arithmetic-total inequality

For B, Sn=n[2(50)+(n−1)5]/2=n(5n+95)/2S_n=n[2(50)+(n-1)5]/2=n(5n+95)/2. Using the source’s rounded target 82258225 gives n2+19n−3290>0n^2+19n-3290>0, with positive cutoff about 48.6448.64. Thus the least positive integer candidate is 4949. Using A’s unrounded total gives the same least integer.

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(ii) Method 2: verify adjacent integer totals

The source calculator table is:

nB total (metres)
488040
498330
508625

These totals increase, and 8040<T<83308040<T<8330, so the least number of days is 4949. The source’s intermediate decimal cutoff is not a fractional permissible day count; round to the next integer satisfying the strict inequality.

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Common mistakes

Use a strict comparison and check whole days; premature rounding must not decide the boundary.

Read the hint