exam Maths Puzzle Numerical Ability

Find the inverse of:
[a b
_c d]

So far I have done this:

[ a b 1 0
__c d 0 1]

~_[ 1 b/a 1/a 0
__cd_01 ]

~_[ 1__b/a_1/a0
0d-(cb/a)__-c/a

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exam Other Question

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Give a counterexample to show the statement is not always true for real numbers a, b, & c. If a margaret is twice as old as jenny, and sarah is twice as old as margaret. Their combined ages total 133 years. How old is each person? in equation form