I found Power Grid to be significantly improved by keeping a calculator on hand for all that addition.

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.