a 2 digit number is 9 times the sum of its digits .if it's ones and tens digits be x and y.write a linear equation in two variable
the number is 10y + x
10y + x = 9(x+y)
10y + x =9x + 9y
y - 8x = 0
or y = 8x
since both x and y must be ≤ 9, the only choice for x = 1
so y = 8
and the number must be 81