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 14 cm: 14 = 8 + 6 adds the diameters; internal touching needs the difference of radii.
• b 7 cm: 7 = 4 + 3 adds the radii; internal touching subtracts them.
• c 2 cm: 2 = 8 − 6 is the difference of diameters; the difference of radii is 4 − 3 = 1.