Question

Using all or some of the digits 2, 3, 4 and 5, Edward writes
numbers greater than 500 without repeating digits. For
example, he might write 543.
How many numbers does Edward write?

Answer 1

Ответ:

__*__*__ = ?

If we count the number of options to "substitute one digit" for each "place" in the number, then we can calculate the answer to this problem.

You can put 2 or 4 in the first place (0 cannot be at the beginning of the number, numbers are less than 500, so 6 and 8 are also unsuitable). Ie 2 options.

2 * __ * __ = ?

You can put 0, 2, 4, 6 or 8 on the second. In theory. We must take into account that the numbers are not repeated, and we have already used one of them.

2 * 4 * __ = ?

Only one of the three unused even digits can be placed in the third place in the number.

2 * 4 * 3 = 24

In total, twenty-four numbers satisfy the condition.

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In fact, it's not hard to guess which: 204, 206, 208, 240, 246, 248, 260, 264, 268, 280, 284, 286, 402, 406, 408, 420, 426, 428, 460, 462, 468, 480, 482, 486.

I found it on the Internet