string a = "8:25";
int
n = 2;//multiply by
string[]
time = a.Split(':');
string
output = string.Empty;
int
c = (int.Parse(time[0]) * 60 + int.Parse(time[1])) * n;
if
(c % 60 == 0)
{ output = (c / 60).ToString(); }
else
output = (c / 60).ToString() + ":" + (c % 60).ToString();
