On ex. 1 python says:
There are 33 integers in that group, they are:
15, 45, 75, 105, 135, 165, 195, 225, 255, 285, 315, 345,
375, 405, 435, 465, 495, 525, 555, 585, 615, 645, 675,
705, 735, 765, 795, 825, 855, 885, 915, 945, 975
See https://www.online-python.com/nxgpkMajEo
On ex. 2:
Turning to mod,
in python (and other programming languages)
% is used as the modulus operator
ab%20 = 17
(a + 10) x (b + 10)%20 =
(ab +10a + 10b + 100)%20 =
ab%20 + 10a%20 + 10b%20 + 100%20 =
17 + 10 + 10 + 0 = 17 + 20 = 37 (assuming a and b not divisible by 20)
But 37%20 = 17
As before, use the onilne calculator, here is the answer:
https://www.emathhelp.net/calculators/algebra-1/polynomial-long-division-calculator/?numer=x%5E10+-+8x%5E8%2B7x%5E4%2B+8x%5E3-12x%5E2+-15x+-+5&denom=+x%5E2+-+1
And an image of the solution:
The fractional expression is the last term in this, so the remainder is -7x -17.
You can use an online calculator for that, the solution, and the calculator can be found at:
https://www.emathhelp.net/calculators/algebra-1/polynomial-long-division-calculator/?numer=5x%5E9+-+4x%5E2+%2B2&denom=5x%2B10
Under is screengrab of the answer:
The answer to Your puzzle is thus:
−644x5+12885−25745x+10
It really is a tedious task to go through all of that.
Here is my doodle
https://www.geogebra.org/classic/bhfnxt4x
Heron's formula, and a calculator for it can be found at https://www.mathsisfun.com/geometry/herons-formula.html