For same quantity of the water in both the tanks, volume of both the tanks must be same.
1) Volume of tank with rectangular base=
$${ V }_{ 1 }=l\times b\times { h }_{ 1 }$$
Where, $$l$$ = length of the rectangular base
$$b$$ = width of the rectangular base
$${ h }_{ 1 }$$ = height of the rectangular tank
$$\therefore { V }_{ 1 }=80\times 70\times 45$$
$$\therefore { V }_{ 1 }=252000\quad { cm }^{ 3 }$$
2) Let $$h$$ = height of the tank with square base
$$a$$ = side of the square base
Thus, volume of the tank with square base =
$${ V }_{ 2 }={ a }^{ 2 }\times { h }_{ 2 }$$
$$\therefore { V }_{ 2 }={ 60 }^{ 2 }\times { h }_{ 2 }$$
$$\therefore { V }_{ 2 }=3600\times { h }_{ 2 }$$
3) $${ V }_{ 1 }={ V }_{ 2 }$$
$$\therefore 252000=3600\times { h }_{ 2 }$$
$$\therefore { h }_{ 2 }=\frac { 252000 }{ 3600 } $$
$$\therefore { h }_{ 2 }=70\quad cm$$