Answer:
number of message sent by Scott is 23
number of message sent by Christine is   30
number of message sent by Milan is 92
Step-by-step explanation:
Total message sent by Christine,Milan, and Scott : 145
Let number of message sent by Scott be x .
Given that Christine sent 7 more messages than Scott
number of message sent by Christine is   x+7
It is also given that  Milan sent 4 times as many messages as Scott
number of message sent by Milan is   4*x = 4x.
Thus, sum of message sent by Christine,milan, and Scott  in terms of x
= x + x + 7 + 4x = 6x + 7
we already know that Total message sent by Christine,milan, and Scott is 145
thus,  6x + 7 should be equal to 145
 6x + 7 = 145
=>6x = 145 - 7 = 138
=> x = 138/6 = 23
Thus,  
number of message sent by Scott is x = 23
number of message sent by Christine is   x+7 = 23+7 = 30
number of message sent by Milan is   4x = 4*23 = 92