21 mango trees, 42 apple trees and 56 orange trees have to be planted in rows such that each row contains the same numbers of trees of one variety only. Minimum number of row in which the trees may be planted is = ?

For the minimum number of rows, the numbers of trees in each row must be the maximum.
Number of trees in each row = HCF of 21, 42, 56 = 7
Hence number of rows
$$\eqalign{ & {\text{ = }}\left( {\frac{{21 + 42 + 56}}{7}} \right) \cr & = \frac{{119}}{7} \cr & = 17 \cr} $$