The mystery number has five digits, the digit in the tenth place is double the digit in the tens place, the mystery number rounds to 700,the difference between the digit in the hundreds place and the ones place is 3

See explanation

Step-by-step explanation:

If the mystery number rounds to 700, then it has 3 digits before decimal points and two digits after decimal point.

Hence, the mystery number is

The digit in the tenth place is double the digit in the tens place, so

If b = 1, then d = 2

If b = 2, then d = 4

If b = 3, then d = 6

If b = 4, then d = 8

The difference between the digit in the hundreds place and the ones place is 3, so

The first digit can be only 7, because b < 5 and the number rounds to 700. So

If a = 7, then c = 4

All possible answers:

714.2e

724.4e

734.6e

744.8e,

where e is an arbitrary digit from 0 to 9.

There are 40 possible numbers which match the situation.