You know that the entire amount is going to draw a min of 7% : so, 70K @ 7% = $4900. Since Mardi made $5900, then an additional $1000 was earned @ 5% ( 12% - 7% = 5%) $20,000 @ 5% = $1000. So, $50,000 is invested @ 7% and $20,000 @ 12%.
$50K @ 7% = $3500 $20K @ 12% = $2400
BTW: Mardi is an idiot and shouldn't be trusted with money.