.
From the condition, you have this general formula
V = ,
where " k " is a constant value.
From the condition, you also have these two expressions for two states of the gas:
120 = (1)
and
100 = , (2)
where P is the unknown pressure under the question.
Divide expression (1) by expression (2) (both sides). You will get
= , or
1.2 = .
which implies
P = = 22.297 atmospheres. ANSWER
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It is the shortest way to solve such problems.
The other approach with calculating the constant value " k " forces you to make unnecessary calculations on the way to the answer.