A radius, from the diameter

Difficulty Standard · Set 5b6r2n

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 38 cm. How long is the radius marked ?, in cm?
2
The circle's diameter is 20 cm. How long is the radius marked ?, in cm?
3
The circle's diameter is 32 mm. How long is the radius marked ?, in mm?
4
The circle's diameter is 4 cm. How long is the radius marked ?, in cm?
5
The circle's diameter is 32 mm. How long is the radius marked ?, in mm?
6
The circle's diameter is 36 mm. How long is the radius marked ?, in mm?
7
The circle's diameter is 22 m. How long is the radius marked ?, in m?
8
The circle's diameter is 36 cm. How long is the radius marked ?, in cm?
9
The circle's diameter is 22 cm. How long is the radius marked ?, in cm?
10
The circle's diameter is 34 cm. How long is the radius marked ?, in cm?
11
The circle's diameter is 10 cm. How long is the radius marked ?, in cm?
12
The circle's diameter is 40 m. How long is the radius marked ?, in m?
13
The circle's diameter is 12 mm. How long is the radius marked ?, in mm?
14
The circle's diameter is 12 mm. How long is the radius marked ?, in mm?
15
The circle's diameter is 4 mm. How long is the radius marked ?, in mm?
16
The circle's diameter is 34 cm. How long is the radius marked ?, in cm?
17
The circle's diameter is 4 cm. How long is the radius marked ?, in cm?
18
The circle's diameter is 26 mm. How long is the radius marked ?, in mm?
19
The circle's diameter is 32 mm. How long is the radius marked ?, in mm?
20
The circle's diameter is 10 mm. How long is the radius marked ?, in mm?

What is in this set

The numbers in this set run from 4 to 40.
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 56 different ones, 23% — 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.