answer:
10
Explanation:
easy: 100 ÷ 10 = 10. you need 10
explanation: you need 100 (on the left side) to equal the right side. each weight on the right side weighs 10, but you don't know how many you need. use x for the unknown amount. now, you have:
100 = 10x. solve by dividing each side by 10.
10 = x. verify by doing 10 x 10 = 100