Final answer:
To determine how many Tartar horde members did not ride horses, five sevenths of the total horde (140,000) is calculated to be riders (100,000), and thus 40,000 did not ride.
Step-by-step explanation:
The question asks how many members of the Tartar horde did not ride horses if five sevenths of them did, given that the total number of the horde is 140,000. To solve this, first find the number of riders by calculating five sevenths of 140,000:
<5/7> * 140,000 = 100,000
Now, subtract the number of riders from the total number to find out how many did not ride horses:
140,000 - 100,000 = 40,000
Therefore, 40,000 members of the Tartar horde did not ride horses.