Their car goes 150 miles _____ hour.
Their car goes 150 miles _____ hour.
Explanation
"Hour" begins with a silent h, so it sounds like it starts with a vowel.
Therefore, we use "an" before "hour" — "150 miles an hour."
"Hour" begins with a silent h, so it sounds like it starts with a vowel.
Therefore, we use "an" before "hour" — "150 miles an hour."