P4 Math: Assumption Method

There are 30 questions in a Science contest. 
6 marks will be awarded for each correct answer.
4 marks will be deducted for each wrong answer.
All the questions must be answered.

If Sakinah scores 100 marks in the Science contest, how many questions does he answer correctly ?

Solution:

We are not using guess and check method, instead we will use assumption method which is also known as TEDO (Total, Excess, Difference, Opposite) method.

T

Let assume total score if Sakinah answered all correctly, means:
30 x 6 =180

E

We can see there is excess of (180 - 100) = 80 marks

D

Marks difference between correct and wrong answer is (6 + 4) = 10 marks.
Please note that it is not (6 - 4) = 2.

O

Opposite, means there are some questions answered wrongly. 
How many of them ?
It can be calculated = Excess / Difference = 80 / 10 = 8.
Hence there are 8 questions has wrong answer.
So there are (30 - 8) = 22 questions has correct answer.

Check:

Total mark = (22 x 6) - (8 x 4) = 100  , Matched !