the number fell 10 percent / "by" 10 percent.


Senior Member
Korea, Korean
Since 1999, the number of undergraduates majoring in sciences and engineering has fallen 10 percent to 503,026. Applications to the engineering program at Utsunomiya University have fallen one-third since 1999. (source)

According to the OALD, fall means:
to decrease in amount, numto decrease in amount, number or strength: [v] Their profits have fallen by 30 percent.

Do you need to put "by" in the example above?
I wonder which one is correct. Maybe both?
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