Let's call the unknown number ' W '.
You said that 100 is 1 tenth of W .
Write that as an equation: 100 = (1/10) W
Now multiply each side by 10 : (10) 100 = (10) (1/10) W
Left side: (10) 100 is 1,000
Right side: (10) (1/10) is 1
So the equation says that 1,000 = W