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If f(x)=3x+1 and f^-1=x-1/3, then f^-1(7)=

The suare root of 9x plus 7 plus the square rot of 2x equall to 7

At the beginning of 1974/75,2 years a man invested #2000 in a bank at 12.5% compound interest, if it degree #650 at the end of each two years to pay his son's school fees, after payment of interest to pay his son's school fees, how much did he have left at the beginning of the 1978/79 school year?