1155
1
27
2
14 x (1 + 4) = 14 x 5 = 70
There are 9 digits that can be the first digit (1-9); for each of these there is 1 digit that can be the second digit (6); for each of these there are 10 digits that can be the third digit (0-9); for each of these there are 10 digits that can be the fourth digit (0-9). → number of numbers is 9 × 1 × 10 × 10 = 900 such numbers.
1
Find a four digit number whose digits will be reversed when multiplied by nine?
Six.
Not necessarily. Consider 444. The digits are not different. The first and second digits are not multiples of 3 The first digit is not greater than the second digit. In spite of all that, 444 is a 3-digit number
A two digit number whose two digits added together equal 42 and multiplied together equal 82
27
The number is 21978. 21978 when multiplied by 4 which gives the result 87912 which is in reverse order.
2.3
2
There are 9 digits that can be the first digit (1-9); for each of these there is 1 digit that can be the second digit (6); for each of these there are 10 digits that can be the third digit (0-9); for each of these there are 10 digits that can be the fourth digit (0-9). → number of numbers is 9 × 1 × 10 × 10 = 900 such numbers.
14 x (1 + 4) = 14 x 5 = 70
Any prime numbers of about 66 or 67 digits will do.