1) Why B could not figure it out when he knew the sum of the ages?
2) The slope of a line is equal to the tangent of its angle with the X-axis,
   try to figure out what tan(t+pi/2) is and to use it to get the answer.
3) Ignited from both ends sinultaneously, how long will a rope burn?
4) Read the handouts.
5) The surface area A will be proportional to H*H, i.e. A=cH*H 
   the equation dH/dt=-(a/A)v(H)=-(a/A)sqrt(2gH) still holds,
   stick the expression for A into it and try to work out the rest.