The following solution was sent to us by Josh (Alameda School)
The easiest way of seeing how it works is to use x as
the first number.
Multiply by 3 makes it 3 x
Adding 6 makes it 3 x + 6
Taking away the start number makes it 2 x + 6
Then dividing by 2 makes it x + 3
Taking away the original number then leaves 3.
Well done Josh!
Wat Zhiwen Sergius (Anglo-Chines Primary School, Singapore) writes the solution this way... "Let n be the number (3n+6)-n = (2n+6) (2n+6)/2 = (n+3) (n+3)-3 = n