Opal has more than 30 but less than 40 chocolate bars. If she puts them equally into bags of 10, she would be short of 9 chocolate bars. If she puts them equally into bags of 8, she would have 7 extra chocolate bars. How many chocolate bars does Opal have?
Multiples of 10: 20, 30, 40, 50
Multiples of 10 (-9): 11, 21, 31, 41
Multiples of 8: 16, 24, 32, 40, 48
Multiples of 8 (+7): 23, 31, 39, 47, 55
Common number more than 30 but less than 40: 31
Number of chocolate bars that Opal has = 31
Answer(s): 31