What are the possible routes from A to D in a 2D grid?

[LCKurtz]

I think part d has an error.

We need to calculate (A to B * B to D) - (B to C * C to D), which equals
(35*3003) - (21*56)
105105 - 1176 = 103,929

That is ALMOST correct. Think about what the 35 should multiply.

 

[cronuscronus]

It looks like we need to calculate (A to B * B to D) - (B to C * C to D * A to B)
so
(35*3003)-(21*56*35) = 63,945 routes for part d.

 

[LCKurtz]

It looks like we need to calculate (A to B * B to D) - (B to C * C to D * A to B)
so
(35*3003)-(21*56*35) = 63,945 routes for part d.

Yes. It might have been better to express it as
(A to B)*(B to D - BtoCtoD)

I think you have it now and hopefully you understand it well enough that you could explain to others how to do it.

 

[cronuscronus]

I am going to explain to a friend tonight. If you're ever in San Diego I'll buy you a beer :)