A radius, from the diameter

Difficulty Starter · Set rpg2m1

The drawings are schematic: work from the lengths marked on them, not from their proportions.

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

1
The circle's diameter is 4 mm. How long is the radius marked ?, in mm?
2
The circle's diameter is 12 m. How long is the radius marked ?, in m?
3
The circle's diameter is 8 m. How long is the radius marked ?, in m?
4
The circle's diameter is 8 cm. How long is the radius marked ?, in cm?
5
The circle's diameter is 16 m. How long is the radius marked ?, in m?
6
The circle's diameter is 14 cm. How long is the radius marked ?, in cm?
7
The circle's diameter is 18 mm. How long is the radius marked ?, in mm?
8
The circle's diameter is 12 mm. How long is the radius marked ?, in mm?
9
The circle's diameter is 18 cm. How long is the radius marked ?, in cm?
10
The circle's diameter is 16 m. How long is the radius marked ?, in m?
11
The circle's diameter is 4 m. How long is the radius marked ?, in m?
12
The circle's diameter is 8 cm. How long is the radius marked ?, in cm?
13
The circle's diameter is 18 mm. How long is the radius marked ?, in mm?
14
The circle's diameter is 6 cm. How long is the radius marked ?, in cm?
15
The circle's diameter is 12 cm. How long is the radius marked ?, in cm?
16
The circle's diameter is 6 cm. How long is the radius marked ?, in cm?
17
The circle's diameter is 18 m. How long is the radius marked ?, in m?
18
The circle's diameter is 14 m. How long is the radius marked ?, in m?
19
The circle's diameter is 6 cm. How long is the radius marked ?, in cm?
20
The circle's diameter is 10 m. How long is the radius marked ?, in m?

What is in this set

The numbers in this set run from 4 to 18.
Every problem in it will run the formula backwards.
A careful solution takes 1 written step on average.
The mistake this set invites most often, in 100% of the problems: the diameter was doubled; the radius is half of it, from the centre to the edge.
Answers are a whole number.

Drawing 240 problems from this page produced 24 different ones, 10% — so a sheet of 20 will come back to a problem you have seen — there are only so many sensible ones of this kind.

The same skill, harder or easier

We would put this in front of a grade 5 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.