The period between about 1650 and 1700 is when astronomers saw virtually no sunspot activity.
Answer:

Explanation:
From the question we are told that:
Density 
Frequency
Length
Generally the equation for Frequency is mathematically given by

Therefore



To reduce the internal energy U of an ideal gas you need the gas to lose heat and/or do work.
The mass of the child in this problem is 24 Kg, the unit for mass is in Kilogram.
The mass will change only if it travels close to the speed of light.