Two circles of diameters 8 cm and 6 cm touch each other internally. What is the distance between their centres?

Correct answer: d 1 cm

Radii are 4 and 3; internal touching gives d = 4 − 3 = 1 cm.

Why the other options are wrong

• a 4 cm: 4 is the radius of the larger circle, not the distance between centres; the smaller radius 3 must be subtracted.
• b 3 cm: 3 is the radius of the smaller circle; the distance is 4 − 3.
• c 2 cm: 2 = 8 − 6 is the difference of diameters; the difference of radii is 4 − 3 = 1.