Very true, factor.
We see questions on here headed "maths" when in fact they are usually matters of simple (often mental) arithmetic. There is, as you say, the issue of "do I multiply or divide?" or "which number do I divide by?". Decimal places and confusion when pence and pounds are mixed seem very common. It seems that people are not taught to think.Very often a simple rough calculation will provide a "ball park" figure. In this particular example I would do something like "If they cost me £3,000 that would mean I'd get 150,000 pens for £3k, so 150 would cost me 300 pence, so they are about two pence each. In most cases that estimate would probably be sufficient, but as has been demonstrated there are various ways to calculate the exact cost.
I'm not having a go at princerupe. My dismay is directed towards an education system that, after a minimum of eleven years (soon to be thirteen) of basic education churns out people who need help with the most simple tasks.
Sorry, princerupe, your question has been somewhat hijacked but I'd be very interested to learn if you have been taught how to tackle questions such as this.