Estimating a difference by rounding

Difficulty Starter · Set lac4ob

Round each number to the nearest 10 first, then work out the difference of the rounded numbers.

Fill in what you can and press Check. Nothing you type is sent anywhere.

1
83−32=
2
85−40=
3
73−30=
4
98−60=
5
62−45=
6
74−66=
7
39−28=
8
89−13=
9
99−67=
10
15−15=
11
42−31=
12
82−67=
13
58−20=
14
68−21=
15
67−43=
16
88−30=
17
78−37=
18
52−32=
19
87−67=
20
49−43=

What is in this set

The numbers in this set run from 10 to 99.
47% of them round up to an estimate above the exact answer.
43% of them round down to an estimate below the exact answer.
A careful solution takes 3 written steps on average.
The mistake this set invites most often, in 90% of the problems: that is the exact difference.
Answers are a whole number.

Drawing 240 problems from this page produced 237 different ones, 99% — so a sheet of 20 is a new sheet almost every time.

The same skill, harder or easier

We would put this in front of a grade 4 class. That is our judgement, not a standard, and it is printed here so you can disagree with it. Everything else we put in that year.