The purpose of this research was to study the mental arithmetic process by considering the time used as a dependent variable and considering even and odd multipliers, low and high multipliers, and problems without higher-decade addition, with one higher-decade addition, and with two higher-decade additions as independent variables. The adults sample was composed of 20 second-year students of the Higher Certificate of Education from Chachoengsao Teachers’ College. Each subject was presented with 48 mental arithmetic problems, one at a time chosen at random, to be solved. These problems were organized according to the independent variables listed above. The time required was recorded by the observer. These data were computed to arrive at the mean in each case and to analize the variances and the trends. The results showed that: (1) The problems without higher-decade addition consumed the smallest amount of time while the problems with one higher-decade addition consumed more time and the problems with two higher-decade additions consumed much more time. The time consumed was significantly different (F = 12.01, p<.01), and the time consumed increased significantly in the form of a linear trend (F = 283.9,p< .01) (2) The low multipliers consumed significantly less time than the high ones (F = 31.4<.01) (3) The even and odd multipliers consumed the same amount of time (F = 4.97, p˃.01). (4) The various multipliers, (viz: 2, 3, 4, 5, 6, 7, 8 and 9) consumed significantly different amount of time (F = 12.53, p<.01), and the time consumed increased significantly in the form of a linear trend (F = 43.36,p<.01), and also in the form of a quadratic trend (F = 16.03,p<.01). This means that the trend of the time increase was not exactly linear but it was still more linear than quadratic (5) There was an interaction between higher-decade additions with low and high multipliers (F = 43.94,p<.01). (6) There was also an interaction between higher-decade additions with even and odd multipliers (F = 8.3,p<.01).
