1) When the shadow of the top of the tree touches the top of the house, tan(18) = (30 - 20)/d, where d is the distance between the tree and the hoouse.

2) If x ansd y are the digits in a number, then the intege is 10x + y or 10y + x. If x+y is divisible by 3, then (10x + y ) = ( 9x + x + y ) is also divisible by 3.