Half-life time of a reaction is time at which reactant concentration becomes half of its initial value.
Half-life of the first order reaction is 20 min. Rate constant can be calculated as follows:

The rate expression for first order reaction is as follows:

initial number of molecules of reactant are
, time is 100 min thus, putting the values to calculate number of reactant at time 100 min,
![0.03466 min^{-1}=\frac{2.303}{100 min}log\frac{[10^{20}]}{A_{t}}](https://tex.z-dn.net/?f=0.03466%20min%5E%7B-1%7D%3D%5Cfrac%7B2.303%7D%7B100%20min%7Dlog%5Cfrac%7B%5B10%5E%7B20%7D%5D%7D%7BA_%7Bt%7D%7D)
On rearranging,

Or,

Therefore, number of molecules unreacted will be 
4. false
5. true
6.valence electrons
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Answer: normal fault
A fault refers to a planar or curved fracture in the rocks of the Earth crust, in which compressional or tensional forces results in the displacement of the rock on the opposite side of the fracture. Fault may vary in the length of the fracture. A normal fault is a fault in which the hanging wall of the rock moves downward to the footwall. It occurs when the two blocks of rock are separated apart by the tensional forces.
On the basis of the above information, the normal fault is the fault that should be found by the geologists in the rock.
The answer is B. They all have backbones. They can all do many tasks and their backbone allows them to do so. All mammals have backbones. Although an alligator is a reptile it still has a backbone