A Ninja Maths Master should be able to estimate how long a walk will take
The average ten year old child will walk at about 4km per hour.
With this knowledge a Ninja Maths Master should be able to work out that 6km is one and a half times 4km, so the time taken will be one and a half times one hour which is one and a half hours!
Ask how long it would take to walk 40km. The answer will not be ten hours. Why?