Answer :
Total amount saved in 1st case :
[tex]S_1=400+20t_1[/tex]
Total amount saved in 2nd case :
[tex]S_2=100+40t_2[/tex]
( Here [tex]t_1 \ and\ t_2[/tex] are number of months )
Let, after x months the amount saved in both cases is same.
So,
[tex]S_2=S_1\\\\100+40x=400+20x\\\\20x = 300\\\\x = 15\ months[/tex]
Amount saved :
[tex]S_1=400+20(15) =\$ 700[/tex]
[tex]S_2=100+40( 15)= \$700[/tex]
Hence, this is the required solution.