A sign in a gas station advertises gas at the rate of $3.99 9/10 per gallon. How many dollars does a gallon cost, writing the price strictly as a decimal?
We can write $3.99 9/10 as a decimal by recognizing that the 9/10 corresponds to the fraction $\frac{9}{10} = 0.9$. Writing the price as a decimal, we have $3.99+0.9 = \boxed{4.89}$.
To convert the price to a decimal, we need to consider the fraction part. There are 10 parts, and the fraction is 9/10.
To convert this fraction to a decimal, we divide the numerator (9) by the denominator (10):
9 รท 10 = 0.9
Therefore, the fraction 9/10 as a decimal is 0.9.
Now, let's add this decimal to the whole number part, which is $3:
$3 + 0.9 = $3.9
Hence, a gallon of gas costs $3.9 when written strictly as a decimal.