Your question: It take 324 kilogram of coal to run a 100 watt lightbulb 24 hrs a day for a year. That equals weight of 3 baby elephant. How many 100 watt light bulbs could run for 12 hrs a day with 11 baby elephant weight of a coal?It is a very good question and much harder to explain to a 10 year old with the given data. Here's my way of explaining. Note that I have used 12 baby elephant weight of coal instead of 11 for ease of calculation.

** Modified question**.

It take 324 kilogram of coal to run a 100 watt lightbulb 24 hrs a day for a year. That equals weight of 3 baby elephant. How many 100 watt light bulbs could run for 12 hrs a day with 12 baby elephant weight of a coal?**Understanding the question:**324 kg coal OR 3 baby elephant equivalent used to burn and provide energey to light "one" 100 watt bulb for 24 hours a day. The question is how many (number of) 100 watt bulb can be lit for 12 hours a day using 12 baby elephant equivalent of coal. The 100 watt of bulb is same & provided just to confuse so you can safely ignore it.

3 baby elephant == to light 1 bulb 24 hours a day

**Step 1 **Now reduce coal to half the quantity. Why?? To make it equivalent to the question i.e. 12 hours a day.

1.5 baby elephant == to light 1 bulb for 12 hours a day

At this step it is the same bulb and same #hours a day in the question asked

**Step 2** Now to the previous step,add 1.5 baby elephant == to light 1 bulb for 12 hours a day which makes

3 baby elephant == to light 2 bulbs for 12 hours a day

**Step 3** Now to the previous step, add 3 baby elephant == to light 2 bulb for 12 hours a day which makes

6 baby elephant == to light 4 bulbs for 12 hours a day

**Step 4** Now to the previous step, add 6 baby elephant == to light 4 bulb for 12 hours a day

which makes 12 baby elephant == to light 8 bulbs for 12 hours a day

With this understanding, let's do the original question:

3 baby elephant == light 1 bulb 24 hours a day

1.5 baby elephant == light 1 bulb 12 hours a day

Therefore 11 baby elephant ==

"b" bulb 12 hours a day

b = (11 X 1)/1.5

**Answer**: 22/3 or 7 1/3

sweethome, maznaq - I hope my explanation helps. Here are some variants of similar question that you can try to test the understanding:[/color]

Q2 It take 3 litres to petrol to run a BMW car for 24 hrs a day. How many days can the BMW car run for 12 hrs every day with 6 litres of petrol?Data given is

3 litres == 24 hours for 1 day

1.5 litres == 12 hours for 1 day

Question is 6 litres == 12 hours for "d" days

d = (6 X 1)/1.5 [Rule of 3]

Answer: 4 days

[Challenge for our DS's]

Q3 It take 4 hours of charging iPhone to play Minecraft for 6 hrs a day. How many days can you play Minecraft for 12 hrs a day by charging iPhone only once for 24 hours ? Data given is

4 hours charging == 6 hours playtime for 1 day

8 hours charging == 12 hours playtime for 1 day

Question is

24hours charging == 12 hours playtime for "d" days

d = (24 X 1)/8 [Rule of 3]

Answer: 3 days

*Any feedback/criticism of the response is really appreciated *

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11 Plus is not difficult to crack. It is all about the way you teach.