1% of 100 balls is 1 ball.

1% of 500 is 500/100 = 5

10% of anything is 10 x 1%.

1% of 1000 tons is 10 tons.
10% is 100 tons.
5% is 50 tons.
1% of 100 ohms is 1 ohm.
5% of 100 ohms is 5 ohms.

A 100 ohm resistor with a tolerance of 5% can have a value between 95 and 105 ohms and be within tolerance.