Thomas of QEGS Wakefield identified the
To light the YELLOW light the rule
is: < $30$
To light the RED light the rule is:
Have an odd tens digit
To light the BLUE light the rule
is: Multiple of $5$
To light the GREEN light the rule
is: Is odd
Ahmad reasoned well to explain the
minimum number which lit up all the lights:
The answer is $15$.
Its tens digit is the smallest, from the allowed, which is
$10$. Its unit digit is $5$ ; it is not zero because then it would
be even and wouldn't light the green. It is also less than $30$.
Are you both sure that the yellow light
only lights up for numbers under $30$?