Alternatively.

Till Got £25 + shopkeeper stole £2 = Men paid £30 - £3 refund = £27

You should see from that that when you add your £2 to the £27 the men paid (with refund), you are accounting for the same £2 twice as has already been accounted for on the left side of the equation. In other words, the question tricks you into adding things together that shouldn't be in search of the £30 answer. If you wanted to arive at £30 balancing, just do a simple bit of algebra on the equation, taking the £3 to the other side.

Till Got £25 + shopkeeper stole £2 + £3 refunded = Men Paid £30