Just adding the digits until you get to a single figure divisible by 3 weeds out many of them. 5+7=12, 1+2=3. You are permitted to omit the last step! So 91 ha

Just adding the digits until you get to a single figure divisible by 3 weeds out many of them. 5+7=12, 1+2=3. You are permitted to omit the last step! So 91 had me wondering for a while.