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A large container holds 14 litres of a solution that is 25% antifreeze, the remainder being water. How many litres of antifreeze must be added to the container to make a solution that is 30% antifreeze?
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.
If the base of a rectangle is increased by 10% and the area is unchanged, by what percentage is the width decreased by ?
Equal circles can be arranged so that each circle touches four or six others. What percentage of the plane is covered by circles in each packing pattern? ...
Prove that the shaded area of the semicircle is equal to the area of the inner circle.