How do you model a decimal using a fraction

Okie dokie, 
When converting a decimal, you use the place the decimal is in...
Let's review the places: tenths, hundredths, thousandths, etc.
You look at the last number to determine what place you're using!
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Now here's an example for the tenths place:
.5
it's in the tenths place, right? so put it over that number (with no decimal) over a 10.
(mobile) 5/10 or 
Now, decide if you can simplify. You can! 5/10 simplifies down to 1/2.
is your answer!
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Example for the hundredths place:
.26
the last number is in the hundredths place, so put the number (without a decimal) over 100!
26/100 or 
You can simplify this!
13/50 is your answer!

Calculators can also come in handy!
Good luck!

If you need an equation, it would be y=2x+8, y would be how long it grew, x being weeks.if you need how long he was asleep, it would be 4 weeks if he started with no beard, the equation would be x=8÷2if he did have a beard before hand, the equation would be 2w+x=8