Question 561720
Sal has $3200 invested in an account paying 7.2% interest per year.
 At what rate must he invest an additional $2800 so that the annual interest from the two investments will be $390? 
:
Let r = rate required for the $2800 to get a total $390 return
I = P*r*t
where 
p = 2800
r = interest rate
t = 1 yr

:
.072(3200) + (2800*r*1) = 390
230.40 + 2800r = 390
2800r = 390 - 230.40
2800r = 159.60
r = {{{159.6/2800}}}
r = .057 ~ 5.7% is the rate required
:
:
Check this
.072(3200) = 230.40
.057(2800) = 159.60
---------------------
total int: = 390.00