Question 427900
If X number of dollars are invested with a return of 150% (1.5 times the principal), then our equation for the four years is:

X+1.5X+2.25X+3.375X+5.0625X=X+19,500
12.1875X=19500
X=1600

So our principal was $1600.  Our interest for the fourth year was 5.0625 times $1600 or $8100.