Have a look here if you want to: How can I start mental calculating - #33 by bjoern.gumboldt
…or just half of 2400 and then adding 3 x 24
…using 100 as a base:
87 -13
99 -1
either across 87 - 1 = 86 or 99 - 13 = 86 or as 100 - 13 - 1 = 86 is your left and side and 1 * 13 is your right hand side.