Question 1201131
high-risk stocks $  amount invested be z ----15%
medium-risk stocks $ amount invested be y-----10%
low-risk stocks $ amount invested be x-------6%
Total investment $400,000

Condition 
x = y+z...................(1)
x+y+z =400000..............(2)

6%x+10%y+15%z =40,000..........(3)

 x+y+z =400000   (x=y+z)

x+x = 400000
x=200000
Therefore y+z =200 000....................(4)

6%x+10%y+15%z =40,000 multiply by 100

6x+10y+15z =4000000
6(y+z)+10y+15z = 4000000
16y+21z = 4000000................(5)
Solv (4) &(5)
5z =800000
z= 160000
y= 200000-160000 =40000

high-risk stocks $  amount invested 160,000
medium-risk stocks $ amount invested 40,000
low-risk stocks $ amount invested 200,000