If the whole clock face is divided into 12 squares, the minute hand will walk 12 squares every hour and the hour hand will walk 1 squares every hour.

At 7 o'clock, the hour hand is 7 squares faster than the minute hand. If the hour hand and the minute hand make an angle of 90 degrees, the distance between them is three squares.

Let t hours pass, and the hour hand and the minute hand make an angle of 90.

| 12t-(t+7)|=3

| 1 1t-7|=3

1 1t-7=3, or 7- 1 1t=3.

Solution: t= 10/ 1 1, or t = 4/1.

Therefore, at 7: 00, 2 1: 48 and 7: 54: 33, the minute hand and the hour hand make an angle of 90.