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Let ( X )be the loan amount with 2.8% interest rate.

Let (31,000-X) be the loan wity 4.5%.

So put these together equation will come out like this

( X * 2.8% ) + ( 31000-X) * 4.5% = 1024.4

X=21800which is the loan amount with 2.8% int rate.

31000-21800 =9200will be the loan amount wity 4.5%.

you can back test with answer as follow

21800 @ 2.8% = 610.40

9200 @ 4.5@ = 414

610.40 + 414 = 1024.4 so answer is correct