Answer:
fire ant = 5 times
crab spider = 4 times 
Step-by-step explanation:
Given 
The size of fire ant is 4
The size of crab spider is 5
Now John wants to magnify the image of both the fire ant and the crab spider so that they appear the same size.
So we know that 
The multiples of four is 4 is 4, 8, 12, 16, 20, 24,...
The multiple of five is 5 is 5, 10, 15, 20, 25,....
Their common multiple is 20. 
So the image of both the fire ant and the crab spider should be magnified to 20 so that they appear the same size.
Therefore, for the fire ant = 20 / 4 = 5 times
                   for the crab spider = 20 / 5 = 4 times  
Thus the fire ant should be magnified 5 times 
          the crab spider should be magnified 4 times 
   so that they the same size of 20.