Viper Stike and Increased Duration

I've look around the forum and i can't seem to find the answer to this.

If i stack increased duration with viper strike how does it work?

For example, the skill tree says 15% increased duration. So if i take those three skill nodes is the duration

base *1.15 *1.15 *1.15 or base * (1+ .15 +.15 +.15)?

Also, does the increased duration gem also stack because that gives 50% increased duration. i.e.

base * 1.15 * 1.15 * 1.15 * 1.5 or base * (1 + .15 + .15 + 15 + .5)

the second for both situations: base *(1+ 0.15*(node count) +support gem)

Thanks. So very hard to get over 10 seconds with this then.