I love powergrid! I recall the last two turns require a lot of calculations. When I played with non-mathematically oriented people (most people in my life), they got frustrated.
It's the #6 game on BoardGameGeek [1], and well deserved to be there. Saying that "it's filled with math" doesn't make it sound all that exciting, but there's a good spacial element, an auction system that gets quite tense at times, and opportunity for "screw you" behavior that actually pays off. The winner is typically the one who was most efficient and has made some clever plays to keep his opponents out of his chosen markets.
How much of my hundred can I bid on this power plant? Well I need four oil this turn at what might be seven cost each, and I want to buy three houses at fifteen, so that means forty five plus twenty eight is seventy three, so I can spend up to twenty seven in this auction. There's no reason to force all that mental arithmetic, and it really sucks if someone loses because they missed carrying a digit mentally. Just use a calculator to abstract away the math and speed things up.