Our teacher told us to find out the time period and then multiply it by 18.Then 180-(∆t*18) gives firing angle.Why 18 as a multiplier?
It would help if you had some dimensional analysis in there.
Presumably (though you didn't bother filling in your profile so it's hard to tell) you're in a 50Hz country, so each half-cycle is 180 degrees and 10 milliseconds. Hmmm.. 180°, 10ms ... anything we could do with those? Maybe to find °/ms?