It would not be fair to tell you!
Basically, it's algebra.
You can use x and y or a and b.
Here's a clue "There are 2 things you do not know"
the amount that 5% interest is earned from and the amount 8% interest is earned from.
What do we know?
They generate $100 interest, which means the sum of the 2 interests is $100.
Also we know that the 2 amounts themselves sum to $1400.
To find 2 values you need 2 clues.
Are you familiar with solving simultaneous equations,
like 2x+5y =7 and 3x-y =4 ??
This is the same.
So what is x+y ?
Also what is x(5/100) + y(8/100) ?
Try to figure that out