I dont think you'll be able to do that without an API script and pro subscription. Roll20 dice cant do conditional effects (this value is a 0 sometimes, and a 10 other times). Sometimes its possible with a very convoluted dice roll structure, but to do it in this case would require doing some things with the whole d100 roll, and different things with the units value, which you can't do at all. Aaron's suggestion will work if you just want to display the units value, and can accept all results of 10, 20, 30, etc will be displayed as 0, and not as 10.