Grass eaten by two cows in 40 minutes =2×40=80 points.
Grass eaten by four cows in 16 minutes =4× 16=64 portions.
80-64= 16, that is, 40- 16=24 minutes, the newly grown grass.
New grass =(80-64)÷(40- 16)=2/3 copies per minute.
Original grass =80-40×2/3= 160/3.
10 minutes of newly grown grass = 10×2/3=20/3 copies.
To eat all the grass in 10 minutes, the number of cows needed = (160/3+20/3) ÷10 = 6.