倍 and 倍数: a size relationship and a multiple condition
Compare a twofold relationship between quantities with the condition of being a multiple, then select qualifying numbers.
Comparing two quantities by a factor
倍, read ばい, expresses a factor. この箱はあの箱の二倍の重さだ。 says that this box weighs twice as much as that box.
If that box weighs three kilograms, this box weighs six in this comparison. 二倍 describes the relation between weights, not a count of two boxes.
A condition on integers
倍数, read ばいすう, means a multiple. 十二は三の倍数です。 says that twelve is a multiple of three. Multiplying three by an integer can produce twelve.
番号が五の倍数なら丸を付けてください。 requests circling a number if it is a multiple of five. It does not request marking every number.
Select the qualifying numbers
In the list 番号は十、十二、十五です。, ten and fifteen are multiples of five. Twelve is a multiple of three but does not meet the stated condition of a multiple of five.
二倍の重さ compares weights; 五の倍数 sets a condition for numbers. Do not turn the weight relationship into the rule for selecting numbers.
Should twelve be circled under the condition 番号が五の倍数なら?
Show the answer and why
No. Twelve is not a multiple of five. In a list of ten, twelve, and fifteen, mark ten and fifteen.