Solution :
The significant figure of a number are defined as the positional notation of that number which are most reliable and are absolutely necessary to represent the quantity of something.
In the context, we have to express the given numbers into three significant figures in the form of scientific notation or in the exponential form :
(i). 5590 -----
(ii). 0.000498 -----
(iii) 135000 -----
(iv) 0.000438 -----