1. "Mum" has scientifically reduced the problem to its essence. Never take the kids to the grocery store if efficiency is your first concern. So go there first.
2. Solve as much of the problem once and for all in advance as possible. Never go to an ATM (if that's what a pay point is). Get a Paypal account, move cash into it online, and use the Paypal card instead of cash. Easier, more secure, eliminates one stop on the trip, and gives you a record of what you spent and where, making budgeting simpler (thus reducing another set of math and tracking problems).
3. Change your prescriptions to three-month refills or better, delivered by mail. There -- two trips out of three to the pharmacy, or all of them, eliminated.
4. The Law of Scientific Parsimony absolutely dictates: Never give a problem to a mathematician if a Mum can solve it. This approach has been proven by experience to work for up to whatever number of children are in your family.

And if you really need to pretend this is a math/geography problem: It is not. It's a management question. The first parameter to plug in is the likelihood of the traveling salesman closing a deal, and how big, at each destination. Send him to the likeliest big deals first and hit the marginal ones up with a phone call. Be very wary of trying to apply math to social-interaction questions with small sample sizes.

Glad to be of assistance.

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