book Maths Puzzle Numerical Ability

Is this corect:if rank of matrix A is <= n-2 than rank of adj(A)=0

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i dont know how to solve this problem...x-1=3
x+4=4 Irma invested some money a 7% and $100 less than that at 8%. Her total interest was $58. How muc did she invest at 7%, rounded to the nearest dollar?