Question

25 trees are planted in a straight line 5 metres about from each other. To water them the gardener must bring water for each tree separately from a well 10 metres. From the first trees line with the trees. How far will he move in order to water all the trees beginning with first, if he starts from the well.

Answer 1

Distance to and fro from the well to
    the 1st tree   =   (10  )* 2 m      = 20 m
    the  2nd tree = (10+1* 5) * 2 m
           :            :
    the  nth tree =   (10 + (n-1)*5) * 2 m 
        :               :
    the 25th tree =  (10 + 24 * 5) *  2 m = 500m

The total distance = sum = (20m + 500m)/2  * 25 = 6,500 m