So:
    

A ratio can be multiplied or divided through by any positive
constant.  So we'll divide it through by 2, so that the 
smallest rate, the boy's, will correspond to 1.

That is  5 : 4 : 2 =  2.5 : 2 : 1.  

Therefore:

    

Suppose 1 boy's work rate is x hectares/day
Then 1 woman's work rate is 2x hectares/day
and 1 man's work rate is 2.5x hectares/day

The combined rate of 2 men, 4 women and 2 boys = 5x+8x+2x = 
15x hectares/day

2 men's rate is 2(2.5x) hectares/day or 5x hectares/day
4 women's rate is 4(2x) hectares per day or 8x hectares/day
2 boys' rate is 2(x) hectares per day or 2x hectares per day

So their combined rate is 10 hectares/10 days or 1 hectare per
 day.

So we set the two equal:

15x = 1
  x =  

So 1 boy's rate is x or  hectares/day,
1 woman's rate is 2x or  hectares/day,
and 1 man's rate is 2.5x or  hectares/day.

Let t = how many days are required

Then in t days, 
4 men can complete 4·t or  hectares
6 women can complete 6·t or  hectares
8 boys can complete 8·t or  hectares.

So the sum of the hectares they have completed in t days must = 16 hectares

    

    

    

2t = 16
 t = 8

Answer: 8 days.

Edwin