Answer
The expression for total interest is $ 0.15x+0.07y $.
Work Step by Step
Given formula is
$ I=Pr $ ...(1)
The first condition is
Principal $ P=x $ and Interest rate $ r=15\%=0.15 $
By using equation (1) the simple interest is.
$ I_{1}=0.15x $
The second condition is
Principal $ P=y $ and Interest rate $ r=7\%=0.07 $
By using equation (1) the simple interest is.
$ I_{2}=0.07y $
Total interest.
$ I=I_{1}+I_{2} $
Substitute values of $ I_{1} $ and $ I_{2} $.
$ I = 0.15x+0.07y$.
