Question 11133
x+4-1+5-3+6-5+7-3+8-6 = 21
x+3+5-3+6-5+7-3+8-6 = 21
x+8-3+6-5+7-3+8-6 = 21
x+5+6-5+7-3+8-6 = 21
x+11-5+7-3+8-6 = 21
x+6+7-3+8-6 = 21
x+13-3+8-6 = 21
x+10+8-6 = 21
x+18-6 = 21
x+12 = 21
 -12  -12

x=9

there were 9 people on the bus when it first started