2. Risa took her periodical tests on several subjects. Her average score not including her Mathematics score is 87. Her average score including her Mathematics score is 86. If her score in Mathematics is 82, how many periodical tests did she take including that in Mathematics?
87 = s/n
86 = s*/(n+1)
s* = s + 82
87 n = s
n+1 = (s+82)/86
n+1 = (87n + 82)/86
86 n + 86 = 87 n + 82
1 n = 4