MTBR Formula error in std.

The following example shows how the MTBR is calculated:

Acquisition date of pump A 01.01.94

First breakdown 10.01.94

Downtime 10 hours

Second breakdown 20.01.94

Downtime 5 hours

Number of breakdowns 2

MTBR = (19 * 24 - 10) / 2 or X + Y (hr) / 2

= (456 - 10) / 2 or (216 + 240 - 10) / 2

= 223 hours

the above explanation is given for calculation of MTBR ,

i believe the explanation is wrong

in the above calculation 10 is being subtracted ,

now my question is how are we getting this 10.

if you try this formula the result is not coming in system through mcjb.

kindly clarify me the same... bit confused