Systems of equation with only 1 equations? So this is a system of equations problem only there's 1 equation as far as I could tell? 
Roberto invested some money at 7% and then invested 2000 more than twice this amount at 10% His total anual income from the two investments was 2900 . How much was invested at 10%?
Equation: $2900 = 0.07x + 2(0.1y + 2000)$

If he invested $x$ money at $7\%$ and $y$ money at $10\%$, then we are told that $$2900=0.07x+0.1y$$ However, we're also told that $y=2x+2000$, two thousand more than twice $x$.
