RTs (Reaction times) for single-digit addition and multiplication problems are different in each of problem types (cf. problem size effect, tie effect), and this pattern is different in two arithmetic tasks: production and verification. The purpose of this study was which factor contributed to differences of RTs if it was assumed that RTs were the function of the associative strength (AS) and the interference strength (IS). AS and IS for each problem type were evaluated by a experimental method (number-matching task), and solution process in arithmetic tasks was examined. Five experiments and a computational simulation were conducted, and next four conclusions were led. Three of the beginning are related to the production task. First, problem size effect is caused by AS in addition, but by IS in multiplication. Second, tie effect is caused by AS in addition, but mainly by IS in multiplication. Third, the phenomenon which multiplication is solved slower than addition is caused by IS. And forth, in the verification task, difference of RTs in each problem type is caused by AS in both addition and multiplication.