Which energy level has the least energy?
n=3
n=1
n=5
n=7

These energy levels are now numbered as n = 1, n = 2 and so forth. These are the principal quantum numbers. Energy level n=1 has the least energy and energy level n=7 the most energy.

Explanation:

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in scientific notation, the value of the  speed of light in a vacuum

  will be written as 2.995 * 10^8. when this value is represented in the form of

  a * 10^b, then, a = 2.995 and b = 8.

the value of the speed of light in a vacuum given above can also be rounded up to approximately 300000000. in this case, the scientific notation will be written as 3 *10^8.