Ateacher needs 70 lengths of string cut to 40 cm each. if balls of string are 10 m long how many balls will be needed

A meter is 100 cm. A ball of string has 10m of string, and you multiply 10m by 100cm, you get 1,000cm. Then you multiply 40cm by 2, and you get 80cm, or two lengths of string. Then you multiply 35, which is half of 70, by 80, and that is 2,800 cm.
2,800÷40cm=70
And 100cm=1m.
The teacher will therefore need 28 balls of string.

Aand b are correctly balanced reactions. they have the same number of atoms in both sides of the reaction