In word (story) problems, the word "of" usually means multiply.
We may write 60% or 60/100 or 0.60 as needed (similar for 40%).
Let x = the sum of money
Translate:
"60% of a sum money is $120" means
(60/100)*x = $120
math:
(60/100)*x = $120
x = $200 [multiply both sides by (100/60)]
Translate again:
"Calculate 40%of the same sum of money" means
(40/100)*($200)
$80- ANSWER
I like (60/100) and (40/100), you might want to do again using 0.60 and 0.40.