Model an inheritance using principal and simple interest

Let $x,y,z$ be the amounts in dollars in the account, bonds and fund. Total principal gives $x+y+z=50000$. Total one-year simple interest gives $0.06x+0.07y+0.08z=3240$. The difference gives $y-z=12000$. These are the three interpretations identified by the source’s equation callout.

The printed calculator solution is $x=30000$, $y=16000$, $z=4000$. Check the principal: $30000+16000+4000=50000$; interest: $1800+1120+320=3240$; difference: $16000-4000=12000$. Thus Stephen invested $30,000 in the account, $16,000 in bonds and $4,000 in the fund.

The lecturer asks what assumption the model makes; the suggested answer is constant interest rates. The source also stresses that models are approximations and simplifications with assumptions and limitations. These fixed one-year simple-interest equations describe the stated model; they do not incorporate changing rates or additional financial effects.