Answer:
 31.5 kilograms of a 5% salt solution and 13.5 kilograms of a 15% salt solution must be mixed together to make 45kg of an 8% salt solution .
Step-by-step explanation:
Let us assume that the number of kilograms of a 5% salt solution used be x .
Let us assume that the number of kilograms of a 15% salt solution used be y .
As given
A 5% salt solution and 15% salt solution must be mixed together to make 45kg of an 8% salt solution .
Equation becomes 
x + y = 45 
5% is written in the decimal form .

= 0.05 
15% is written in the decimal form .

= 0.15 
8% is written in the decimal form .

= 0.08
Thus 
Concentration of 5% salt solution × Number of kilograms of 5% salt solution used + Concentration of 15% salt solution × Number of kilograms of  15% solution used = Concentration of 8% salt solution × Number of kilograms of 8% salt solution
Thus 
0.05x + 0.15y = 0.08 × 45 
Simplify the above

5x + 15y = 360 
Thus two equations becomes 
x + y = 45 
5x + 15y = 360 
Multiply x + y = 45 by 5 and subtracted from 5x + 15y = 360 .
5x - 5x + 15y - 5y = 360 - 225 
10y = 135 

y = 13.5 kilograms 
Put value of y in the equation 
x + 13.5 = 45
x = 45 - 13.5 
x = 31.5 kilograms 
Therefore the 31.5 kilograms of a 5% salt solution and 13.5 kilograms of a 15% salt solution must be mixed together to make 45kg of an 8% salt solution .