Three balls will be needed. You will need 70 pieces of the 40 cm string so 70 multiplied by 40 equals 2800. Next you need to convert the 2800 cm into meters and 2800cm= 28 meters, so if each ball of string is 10 meters long you will need 3 of them which will give you 30 meters so you will have enough with a little extra. Hope this helped!
