Need a little help, Im trying to figure out a way to show the number of days since the last activity on a lead, but be able to reset the counter when the lead status changes.

Im familiar with the formula today() - lastactivitydate which returns the days since the last activity was logged but I need to take it a step further to reset the counter everytime the lead status changes

Hi guys help me out with some concept,
I learnt Set, Map and all their releted methods.
but while writing triggers I cannot decide when to use Map and Set,
please tell me how to decide, while writing, now their is need of Map.

Hi Everyone I have three Custom Objects 
CustomObj A
Custom Obj B
Custom Obj C

CustomObj A is the lookup on Custom Obj B and Custom Obj C 
But Custom Obj B and Custom Obj C in between there is no relation between us Now My Requirement is there is a Number fields on Custom Obj B and Custom Obj C  I want to SumUp of both these fields and Populate on CustomObj A So How to achieve this requirement Please tell me