I'm required to use the Voltage Divider Rule to find the resistance of the Voltage Source, an ammeter, and a voltmeter in the circuits respectively below:

The first problem I'm having is trying to determine what the resistance is at Ri for the first circuit. I discussed this with a friend of mine and he gave me a formula but it didn't make sense. How can I use the Voltage Divider Rule to find the resistance of Ri?I read this part of the textbook which helped: http://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-impact-measured-circuit/

However the link above didn't explain how I can find the resistance I need to solve the problem.

Any help is appreciated! Thanks!