There are $$60$$ seconds per minute and $$60$$ minutes per hour.
Multiplying these will give you the number of seconds per hour.
$$ \dfrac{60\ sec}{min} \times \dfrac{60\ min}{hr} \Rightarrow \dfrac{60\ sec}{min} \times \dfrac{60\ min}{hr} \Rightarrow \dfrac{60\ sec \times 60}{hr} = \dfrac{3600\ sec}{hr} $$
We can now multiply this by $$7$$ hours to find the number of seconds in $$7$$ hours :
$$ \dfrac{3600\ sec}{hr} \times 7h \Rightarrow \dfrac{3600\ sec}{hr} \times 7 hr \Rightarrow 3600\ sec \times 7 \Rightarrow 25200\ sec $$
There are $$25,200$$ seconds in $$7$$ hours.