the left out mass \((500 + 400) \,gm\) oscillates with a period of \(3\, sec\)
\(3 = t = 2\pi \sqrt {\frac{{(500 + 400)}}{k}} \)…...\((i)\)
When \(500\, gm\) mass is also removed, the left out mass is \(400\, gm.\)
\(t' = 2\pi \sqrt {\frac{{400}}{k}} \)…..\((ii)\)
==> \(\frac{3}{{t'}} = \sqrt {\frac{{900}}{{400}}} \) ==> \(t' = 2\sec \)