Questions

Two series alternate here, with every third number following a different pattern.
In the main series, 4 is added to each number to arrive the next.
In alternate series, 5 is subtracted from each number t arrive the next number.
Here in the main series 
3 + 4 = 7
7 + 4 = 11
11 + 4 = 15
15 + 4 = 19

In alternate series 
24 - 5 = 19
19 - 5 = 14...

The logical answer for a puzzle is that the letter 'W'.

A rainbow is a spectrum of light that appears in the sky when sunlight is refracted through raindrops or other drops of moisture in the Earth's atmosphere. They have inspired much popular folklore, including the belief that a leprechaun can be found with a pot of gold at the end of a rainbow.

It is ABACUS.

Words formed are as below 

BUS
CAB
CUB
US

A tetrad is best described by Four chromatids. It is formed from two homologous chromosomes, each consisting of two chromatids in the first stage of meiosis called prophase1.

It will stop at a height where the weight of the balloon with contents is equal to the weight of air displaced by it, i.e , when the upthrust is equal to the downthrust. The question of bursting will depend on how much difference of pressure inside and outside , the material of the balloon can withstand. It is assumed the balloon is filled with a lighter than air gas like hydrogen or helium.

To produce controlled energy i.e energy at a controlled or steady rate, the number of fissions per second must be controlled and maintained at that rate. Excess neutrons should be absorbed by control roads like cadmium.