#1%# simple interest means, a deposit of #$100# will earn #$1# in one year
Hence #6%# simple interest means, a deposit of #$100# will earn #$6# in one year
and so at #6%# simple interest a deposit of #$500# will earn #$6xx5=$30# in one year (as there were no withdrawals).
and at #6%# simple interest a deposit of #$500# will earn #$30xx3=$90# in three years.
In short if #P# is Principal, #r# rate of interest (in % per annum) and time is #t# years,
Simple Interest is #(Pxxrxxt)/100#, if there are no withdrawals.
Hence interest here is #(500xx6xx3)/100=90#
