Okay so here is how it works:
Because 2:3:5 is the ratio 10 is the total because 2+3+5 is 10 right?
You therefore equate 10 to 4000
10=4000
so if 10 is equal to 4000 what about 2,3 or 5?
2/10×4000=800
3/10×4000=1200
5/10×4000=2000
Hope this anwered your question