You will be given addition problems with one of numbers missing, such as 18 + ___ = 24. Break the problem into 2 small parts so the problem can easily be completed as a 'mental math' problem.
18 + ___ = 24 Break the problem into 18 + ___ = 20 and 20 + ___ = 24. For 18 + ___ = 20, the 'missing' number is 2. For 20 + ___ = 24, the 'missing' number is 4. Together, 2 + 4 = 6, so the 'missing' number for 18 + ___ = 24 is 6. Easy 'mental math' steps!
___ + 19 = 23 Break the problem into 2 easy parts: ___ + 19 = 20 and 20 + ___ = 23. The two 'missing' parts are 1 and 3, and 1 + 3 = 4, so the 'missing' part for ___ + 19 = 23 is 4. Easy 'mental math' steps!
Strive to be able to find the sum of the 'missing' parts, then strive to increase your speed. Good luck and enjoy the challenge!