sembgal:

maltbibi:

[quote=\"jolenekoh\"]Please help... How to draw model for this question or table method?


There is a total of 12 cows and ducks on a farm. They have 38 legs altogether. How many cows are there on the farm?

Appreciated

Hi.

Can we solve this using model?

Thanks.

Can only use Guess and Check method. Draw Guess and Check table to solve.
Answer: 7 cows , 5 ducks.
Tip: Average the animals. Take 12/2 = 6
Try 6 cows, 6 ducks first.
6 cows x 4 legs = 24 cows' legs
6 ducks x 2 legs = 12 ducks' legs
Total legs: 24 + 12 = 36 legs (wrong) x

Try again: this time to increase number of legs, presume 1 more cow and 1 less duck.

Now try 7 cows, 5 ducks
7 cows x 4 legs = 28 cows' legs
5 ducks x 2 legs = 10 ducks' legs
Total legs: 28 + 10 = 38 legs (correct)

Final answer: 7 cows[/quote]Another way that I read from my son's Math Olympia book, also called chicken and rabbit problem.

Step 1 - Assume they are same animals with the smallest number of legs. In this case, assume they are all ducks.
Step 2 - with all being ducks, total number of legs are: 12X2=24
Step 3 - there are 38-24=14 extra legs.
Step 4 - The difference of legs between cow and duck is: 4-2=2
Step 5 - use the result from step 3 and 4: 14/2 = 7, so there are 7 cows.
Step 6 - 12-7=5, there are 5 ducks.

To verify the result, 7X4+5X2=38 legs.