Answer :
we name her balance B so we have
B x1.7%=20.40
B x 1.7/100=20.40
B x 1.7=20.40 x 100
B x 1.7=2040
B=2040/1.7
B=1200
Maybe we could substract the amount that she paid for the interest so her balance may be 1200-20.40=1179.60 instead of 1200
B x1.7%=20.40
B x 1.7/100=20.40
B x 1.7=20.40 x 100
B x 1.7=2040
B=2040/1.7
B=1200
Maybe we could substract the amount that she paid for the interest so her balance may be 1200-20.40=1179.60 instead of 1200