Carlos invested part of $3,000 in a 10% certificate account and the rest in a 6% passbook account. The total annual interest from both accounts is $270. How much did he invest at 6%?

(3000-n) * (10/100) + n * (6/100) = 270

300 - 0.1n + 0.06n = 270

300 - (0.1n - 0.6n) = 270

300 - 0.04n = 270

-0.04n = - 30

n = - 30/-0.04 = 750

Investment at 6% = 750

Investment at 10% = 3000 - 750 = 2250

And to check just plug it back in so,

(0.06*750) + (0.1*2250) = 270