I'm not impressed with their explanation.
Q: Shawn invested one half of his savings in a bond
that paid simple interest for 2 years and received $550 as interest.
He invested the remaining in a bond that paid compound interest, compounded annually,
for the same 2 years at the same rate of interest and received $605 as interest.
What was the value of his total savings before investing in these two bonds?
Let be the annual interest rate for both accounts.
Let be the amount invested in each account.
He invested dollars at percent simple interest for 2 years.
. . This earned: . dollars in interest.
So we have: . .
He invested dollars at percent compounded annualy for 2 years.
. . This grew to: . dollars in two years.
His interest is: . dollars.
. . So we have: . .
Substitute  into : .
. . . . . .
Substitute into : .
Hence, he invested in each account.
Therefore, he invested a total of