As you know, all non-zero digits are significant, so right from the start, you know that this measurement has at least
#0.00078 " "-> " " "two non-zero digits: " 7,8#
Now, your goal is to figure out if any of the four zeroes are significant.
In this regard, you can use the fact for numbers that are
If the zeroes follow the decimal point directly, then they are not significant.
In your case, you have
#0.00078 < 1#
so you must at the three zeroes that follow the decimal point. Since they do not follow any non-zero digits, you can say that they are not significant.
#color(black)(0. color(red)(000)78)" " -> " " "three non-significant zeroes"#
Similarly, for numbers that are
#color(blue)(0) .color(black)(00078)" " ->" " "a non-significant placeholder zero"#
Therefore, you can say that your measurement has