There are 156, 208 and 260 students in
groups a, b, c respectively. buses are to be hired to take them for a field
trip. find the minimum number of buses to be hired if the same number of
students should be accommodate in each bus.

156 = 2·2·3·13
208 = 2·2·2·2·13
260 = 2·2·5·13 
      = 2·2·13
      = 52
their gcf is 52, in 156 there are three 52, in 208 there are four 52, and in 260 there are five 52. So that there are minimum of twelve buses needed to be hired.
12
:)

Ais the answer to your ques