f := y -> (-0.07) * (y - 20):

EulerMethod := proc(f, start_time, end_time, y0, h) # y0: initial value  #h: step size
	local cur, y, rate:
	cur := start_time;
	y := y0;
	while cur <= end_time do
		printf("%g %g\n", cur, y);
		cur := cur + h;
		rate := f(y);
		y := y + h * rate;
	end do;
	return y;
end proc:

# step size = 2
printf("Step Size = %a\n", 2);
EulerMethod(f, 0, 100, 100, 2);

# step size = 5
printf("\nStep Size = %a\n", 5);
EulerMethod(f, 0, 100, 100, 5);

# step size = 10
printf("\nStep Size = %a\n", 10);
EulerMethod(f, 0, 100, 100, 10);