3.) London Bridge is falling down
London Bridge will collapse in 17 minutes. 4 people want to cross it before it will collapse. It is a dark night and there is only one torch between them. Only two people can cross at a time.
“A” takes a minute to cross.
“B” takes 2 minutes
“C” takes 5
and “D” takes 4 minutes
How do they all cross the bridge before it collapses?
a and c across .. a comes back .. a and d cross .. and a comes back … a and b cross
thats all them across 
= 12mins