Of three positive numbers the product of the first and second is 42, that of the second and third is 56 and that of third and first is 48, The third number is

Let first, second and third numbers be x , y and z, respectively.
Then , xy = 42.................(i)
yz = 56 ..................(ii)
xz = 48 ....................(iii)
Multiplying Eqs ( i), (ii) and (iii), we get
(xyz)² = 42 x 56 x 48
⇒ (xyz)² = 112896
⇒ xyz = 336 ................(iv)
Dividing Eq. (iv) by Eq. (i), we get
xyz/xy = 336/42 ⇒ z = 8