Waking ³/₄ of his normal speed, Rabi is 16 minutes late in reaching his office. The usual time taken by him to cover the distance between his home and office:

Explanation:-

1^st method:

⁴/₃ of usual time = Usual time + 16 minutes;
Hence, ¹/_3^rd of usual time = 16 minutes;
Thus, Usual time = 16*3 = 48 minutes.

2^nd method:

When speed goes down to
³/_4^th (i.e. 75%) time will go up to ⁴/_3^rd (or 133.33%) of the original time.
Since, the extra time required is 16 minutes; it should be equated to ¹/_3^rd of the normal time.
Hence, the usual time required will be 48 minutes.