Answer:
 7revolutions
Step-by-step explanation:
Given parameters: 
Initial revolution = 250rpm 
Final revolution = 150rpm 
Time = 4.2s 
Unknown: 
Number of revolutions that occur at this time = ? 
Solution: 
To solve this problem; 
 let us find the change in revolution = 250rpm - 150rpm = 100rpm 
Convert the time to seconds; 
 60s makes 1 minute 
 4.2s will make 
 = 0.07min
 = 0.07min 
So; 
 The number of revolutions at this time = 100rpm x 0.07min
 =  7revolutions