You have two bags, four red balls and four white balls. You must
put all the balls in the bags although you are allowed to have one
bag empty. How should you distribute the balls between the two bags
so as to make the probability of choosing a red ball as small as
possible and what will the probability be in that case?

At Holborn underground station there is a very long escalator. Two
people are in a hurry and so climb the escalator as it is moving
upwards, thus adding their speed to that of the moving steps. ...
How many steps are there on the escalator?

Concrete Calculation

Age 14 to 16 Challenge Level

You might like to imagine cutting the mound of earth into a
cuboid, four prisms and four little pyramids in the corners. You'll
need the formula (Area of base x Height)/3 for the volume of a
pyramid.