\(\Rightarrow y_1 = 0, \,y_2 = 1 \,cm \,\,\,\Delta t_1 (1^2 -0^2)\)
\(\Rightarrow y_1 = 1 \,cm, y_2 = 2 \,cm \,\,\,\Delta t_2 (2^2 -1^2)\)
\(\therefore \frac{{\Delta {t_1}}}{{\Delta {t_2}}} = \frac{1}{3}\)
\(\Rightarrow \Delta t_2 = 3 \times \Delta t_1 = 3 \times 7 = 21 \,hrs\)