A Bat, Flying Due East at 2 M/s, Emits a Shriek

Short Answer

A bat, flying due east at 2 m/s, emits a shriek that is reflected back to it from an oncoming insect flying directly toward the bat at 4 m/s. The insect is 20 m from the bat at the instant the shriek is emitted. Sound travels 340 m/s in air. After what elapsed time does the bat hear the reflected echo?