What is the smallest square number that is divisible by $28,40,70$?

2|28,40,70

2|14,20,35

2| 7, 10,35

5| 7, 5,35

7| 7, 1, 7

1, 1, 1

Smallest number divisible by $28,40$ and $70=$ L.C.M. of $28,40$ and $70=2\times 2\times 2\times 5\times 7=280$

So, the smallest perfect square which is divisible by $28,40$ and $70$ is $(2\times 2\times 2\times 5\times 7)\times 2\times 5\times 7=280\times 2\times 5\times 7=19600.$