Type K query

[richmond]

Can anyone help me with this. I do not understand the answer:

Its from the IPS additional questions book, Type K question 7.

(17 [66] 50) (20[48]44) what goes in the space (35 [ ] 60).
The answer is 50 and I cannot see why to explain to DD.

Thanks

[ian35mm]

Second number (60) minus first number (35) equals 25.

25 times 2 equals 50!

[richmond]

Many thanks!