The most simplest way to explain this is by breaking down the 60%.
Think about it.
If 60% space = 30 bikes
then 30% space = 15 bikes
then 10% space must be 5 bikes.
So we now know for sure 60% space = 30 bikes.
And 10% space = 5 bikes.
Then we know remaining 40% space = 20 bikes.
So total space = 100% space, for 50 bikes.
