This should be pretty simple algebra once you know how to set it up. I'll show you that and if you still need help, let me know.
Two equations, two unknowns.
Some amount, call it x, will earn 7 percent. Another amount, call it y, will earn 8 percent. and together those amounts of interest earn need to add up to $1520. So:
0.07*X + 0.08*y=1520
Also we know that together x and y have to add up to $20,000. So:
X + Y = 20000.
Two equations two unknowns. Go to it! (might i suggest the "substitution method where you plug y=20000-x into the first equation.