To determine the investment at each rate:
A total of $7000 is invested:
part x at 6% and the remainder 7000-x at 10%. How much is invested at each rate if the annual interest is $520
6% = 6/100 = 0.06
10% = 10/100 = 0.1


x = $4500 -> invested at each 6% rate
while
7000 - x = 7000 - 4500 = $2500 ->invested at each 10% rate
Therefore the invested at each 6% rate = $4500 and invested at each 10% rate = $2500