⛰️ Rounding Hill

Today we are learning to round numbers! The number line turns into hills, and the ball rolls down into the nearest valley. Let’s watch!

Picture a flat number line. Now let’s turn it into hills. The tens (30, 40, 50, 60) sit in the valleys. The number exactly halfway between two tens (35, 45, 55) is the very top of the hill.

47 is between 40 and 50, but it is closer to 50. The ball is on the right side of the hill, so it rolls down into the 50 valley: 47 rounded to the nearest ten is 50. 43 is closer to 40, so it becomes 40.

We can find it without drawing a hill! To round to the nearest ten, we look at the ones digit. If it is 0, 1, 2, 3 or 4, we round down. If it is 5, 6, 7, 8 or 9, we round up. In 47 the ones digit is 7: up, to 50.

With hundreds the hill is bigger: 300 and 400 are in the valleys and 350 is at the top. 342 is on the left side of the hill, closer to 300. To round to the nearest hundred, we look at the tens digit: it is 4, so we round down. 342 becomes 300.

45 is right at the top: it is 5 steps from 40 and 5 steps from 50. The rule we use is: the halfway number rounds up. If the digit we look at is 5, round up! 45 becomes 50. And 350 rounded to the nearest hundred becomes 400.

Why do we round? For a quick estimate! The book costs £38 and the teddy bear costs £47. If we round, 40 + 50 = 90. The total is about £90, so the £100 in our pocket is enough! The real total is £85, so our estimate is very close.

Careful, this is a common mistake! Rounding 348 to the ten (350) first, and then to the hundred (400), is wrong. Round only once, looking at the original number: 348 has a 4 in the tens place, so we round down. The right answer is 300.

Summary: to round to the nearest ten, look at the ones digit. To round to the nearest hundred, look at the tens digit. If it is 0, 1, 2, 3 or 4, round down. If it is 5, 6, 7, 8 or 9, round up. The halfway number goes up. Round only once! Now try it in Place Value Lift!