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mr. smith dad is 5 times as old as he was 40 years ago. how old is mr. smith

I need to know how to solve this algebra question.  mr. smiths dad is 5 times as old as he was 40 years ago.  how old is mr. smiths dad?

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Vivian L. | Microsoft Word/Excel/Outlook, essay composition, math; I LOVE TO TEACHMicrosoft Word/Excel/Outlook, essay comp...
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Hi Tom;
I ADDED ANOTHER SOLUTION.  SEE BELOW...
x=Mr. Smith's Dad's age 40-years-ago.
 
5x=x+40
 
Let's isolate x...
Let's subtract x from both sides...
-x+5x=x+40-x
4x=40
Let's divide both sides by 4...
4x/4=40/4
x=10
10+40
Mr. Smith's Dad is 50-years-old.
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You could also do...
x=Mr. Smith's age
 
x/5=x-40
Add 40 to both sides to isolate x...
(x/5)+40=x-40+40
(x/5)+40=x
Subtract (x/5) from both sides
-(x/5)+(x/5)+40=x-(x/5)
40=x-(x/5)
Isolate x...
40=x(1-(1/5))
Because (1-(1/5))=0.8...
40=0.8x
Divide both sides by 0.8...
40/0.8=(0.8x)/0.8
50=x