A conceptual explanation: John's making at least 5% (5,000 is 5% of 100,000) interest. An equal amount invested at 4% and 6% would result in an average interest rate of 5%. Any more than half that 100k invested at 4% would bring that average down to below 5% so his interest would be less than 5,000. Thus, John must be investing at least half his money at the 6% rate, so D.

GMAT chooses numbers for a reason, often to deliver a short cut of a solution approach to the savviest test takers. It's no coincidence that those exact numbers are chosen in this problem.