There are clocks A and B. The hands of clock A move normally as clockwise, while in clock B (due to reverse connection), they move anticlockwise. Initially, the two hands of both clocks are at mark showing 12. If after some time, the angle between the directions of two hour hands is 90° (for the first time), then at the same instant, the angle between the directions of minute hand will be

The hour hand of clock A moves clockwise at 0.5 degrees per minute, and the hour hand of clock B moves anticlockwise at 0.5 degrees per minute. If they start at 12, their positions at time t are 0.5t and -0.5t. The angle between them is |0.5t - (-0.5t)| = |t| = 90 degrees. At t = 90 minutes, the minute hand of A is at 90 * 6 = 540 degrees (mod 360) = 180 degrees, and the minute hand of B is at -540 degrees (mod 360) = 180 degrees. Thus, the angle between them is 0 degrees.