Answer:
3
Explanation:
A ball is 10 meters. Since 100 centimeters are in a meter, each ball has 100*10 = 1000 centimeters.
The teacher needs 70 strings each 40 centimeters. The total amount they need is 70*40 = 2800 centimeters.
Since a ball has 1000 centimeters, 2800 centimeters will need 3 balls for a total of 3000 centimeters.