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assuming that by "same amount of money" you mean what he had after Tom gave him some, then
k+t = 40
k - 3/7 k + 1/6 (t + 3/7 k) = 5/6 (t + 3/7 k)
or, more simply,
7t = 3k
so
Ken had $28 and Tom had $12
check: Ken gave Tom $12
\then Tom gave Ken $4
and the both wound up with $20