16, 25, 34, 43, 52, 61, 70
Seven of them.
108,117,126,135,144,153,162. 171, 180
To find how many two-digit numbers have digits whose sum is a perfect square, we first note that the two-digit numbers range from 10 to 99. The possible sums of the digits (tens digit (a) and units digit (b)) can range from 1 (1+0) to 18 (9+9). The perfect squares within this range are 1, 4, 9, and 16. Analyzing each case, we find the valid combinations for each perfect square, leading to a total of 36 two-digit numbers whose digits sum to a perfect square.
1, 3, and 5.
108 117 126 135 144 153 and 162
In base 10, all numbers whose last digit is a 5 or a 0
108,117,126,135,144,153,162. 171, 180
Ten.
There are 5460 five digit numbers with a digit sum of 22.
1, 3, and 5.
108 117 126 135 144 153 and 162
In base 10, all numbers whose last digit is a 5 or a 0
The numbers are 28 and 29.
715 and 777
450.
-21
They belong to a set of numbers whose first digit is 8. There are 720 such numbers and I regret that I do not have the patience to list them all.
5000, 6000, 6491