First you go the tenths place which is the number 9 stands. You look to the left of the number. Here is the tricky part if the number is under 5 for example 1 2 3 4. The number stays the same and everything behind that number is 0. If the number is 5 or higher the number goes up one and everything behind it is 0. So 3.987 rounded to the nearest tenth is 4.000
