This blog is managed by Song Hock Chye, author of Improve Your Thinking Skills in Maths (P1-P3 series), which is published and distributed by EPH.

## Friday, September 05, 2008

### Catholic High Sch 2006 PSLE Math Prelim Q27

Kenneth wants to rent a bicycle from Jim’s Bicycle Kiosk. The cost (C) of renting a bicycle for d hours is given by C = \$(3d + 5). Find the cost of renting the bicycle if Kenneth wants to cycle for 4 hours.

Solution

d hours ----- \$(3d + 5)
1 hour ----- \$(3d + 5)/d
4 hours ----- \$4(3d + 5)/d (Answer)

keentolearn said...

Hi Mr Song

Under examination conditions, probably some students may make the mistakes of substituting 4 for d and arrive at an answer of 17.

At first glance, the answer appears as such.

Must learn to read the question carefully before putting pen to paper.

Anonymous said...

why can't we juz let d=4? isn't d a variable?

Bush Fires said...

Hi

If the question is phrased as The cost for renting a bicycle is given by C=(3d+5) where d is the number of hours, then to substitute 4 for d is correct.

Look at this question from this perspective
If it's 3 hours, then the rental is (3*3+5)
For 1 hour, the cost is (3*3+5)/3
For 4 hours, the cost is 4(3*3+5)/3

If it's 5 hours , then the rental is (3*5+5)
For 1 hour, the cost is (3*5+5)/5
For 4 hours, the cost is 4(3*5+5)/5

Anonymous said...

This is an algebraic qn which i got from an assessment book:
Oil is leaking from a bottle through a crack. After t minutes, the volume, V, of the oil left in the bottle is given by V=40(90-5t) cm3. How much oil will be left in the tank after 12 min? Give your answer in litres.

The solution given by the book is juz substitute 12 into t and u'll get the answer 12 litres. I thought the way this qn is being phrased is juz like the catholic high qn. How come they can juz subs t=12 into the expression? I'm still so confused :(

xiao feng said...

Hi
Anon

Read the original question(CH) and compare with Bush Fires's post.Try to see the difference.

Your question is the same as the question which had been rephrased by BF.

Anonymous said...

i still can't c the difference :(

Anonymous said...

d is the hours.

so, for 4 hours,

\$(3x4+5)=17

why can't it be \$17?