Why do we divide the centuries by $400$ to find whether it is leap year or not?

Check whether the following years are Ordinary or Leap year?

$1994, 1985, 2000, 2007, 2010, 2100$

Convert $20$$\mathrm{minutes}$ into $\mathrm{seconds}$.

Convert $5$ $\mathrm{hours}$ $35$ $\mathrm{minutes}$$40$ $\mathrm{seconds}$ into $\mathrm{seconds}$.

Convert $3\frac{1}{2}$ $\mathrm{hours}$ into $\mathrm{minutes}$.(Write the final answer only in number without unit)

Convert $580$ $\mathrm{minutes}$into $\mathrm{hours}$.

Convert $25200$ seconds to hours.