Voltage Divider with two current sources
Q) What do you know about current sources?

A) That the current at their terminals is always the same as the number written beside them.

Q) So what is the current through the top 1k resistor, whether the switch is open or closed?

A) As it's connected only to the 10mA source, it's always 10mA.

Q) So where does the extra 5mA flow when you close the switch?

A) The only place it can flow ...