In order to convert a fraction into a decimal, you can do one of two ways. I'll do the manual way, first since that's probably what your teacher would prefer.
First, find something that will multiply in the denominator that will equal 10,100,1000.
In this case, it will be 1,000.
1,000 ÷ 8 = 125.
So, you will multiply both the numerator and the denominator by 125.
3 x 1,000 = 375
8 x 125 = 1,000
Now, you have a BIG fraction of 375/1,000 but it's OKAY! Why? You're almost done.
Take your numerator of 375. Since your denominator has 3 zeros in it, you'll be moving your decimal 3 places to the right (from the imaginary decimal in front of the 3). So, we'll have 0.375.
So, your decimal of 3/8 is 0.375.
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Now, for the easiest way!
You will simply divide 3 ÷ 8 = 0.375