The ratio of the second group to the third group is 4:5= 12: 15.
One group: two groups: three groups =8: 12: 15.
Suppose a group of 8x people, two groups of 12x people and three groups of 15x people.
8x+ 12x+ 15x = 140
X=4 people
The first group =8×4=32 people.
The second group = 12×4=48 people.
Three groups = 15×4=60 people.
I wish you progress in your study!
Please ask questions if you don't understand, and adopt them in time if you understand! (*^__^*)