Maths...

Let the cost price be x.
x(1+125%)=900
x(2.25)=900
x=400
.
. . The cost price is $400

EXPLAINATION:
A mcrowave oven is marked at a price 125% more than its cost price.
So "cost price"+"cost price"x125%="marked price"

ALTERNATIVE SOLUTION:
The cost price
=900 : (1+125%)
=900 : 2.25
=$400

HOPE I CAN HELP YOU ^_^

2015-01-02 13:27:21 補充：
sor, 我打錯字：

The cost price
=900除(1+125%)
=900除2.25
=$400

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"Marked price 125% more than it cost price" means :
If the cost price is 100%, the marked price is (100% + 125%).

Let $y be the cost price of the microwave oven.

y (100% + 125%) = 900
2.25y = 900
y = 400

Hence, the cost price of themicrowave oven is $400.

2015-01-09 14:59:21 補充：
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退一步，用同情、憐憫之心看待，生活會過得更平靜、安心。共勉之。