Question 573123
Lets use X for the number we're looking for.  


5 times the sum ... means we need to figure the sum first and then multiply by 5.
So, 
the sum of X and 11 is X + 11


Take 5 times that ... 
5 (X + 11) 


is 15.  Means equals 15.
5 (X + 11) = 15


Multiplying out ... 
5X + 55 = 15
Subtracting 55 from each side ... 
5X = 15 - 55 = -40
5X = -40
Dividing each side by 5 ... 
X = -8


Checking:
5 times the sum of -8 and 11 is 15.
5 times (-8+11) is 15
5 times 3 is 15
15 = 15  Correct!