Astudent measures the volume of a liquid and finds it to be 40 ml how many significant figures are in the measurement

The number of significant figures in the reading is one.

Significant figures are used to represent the accuracy of a reading. The greater the number of significant figures, the greater the accuracy that is involved. To the left of the decimal point, the first significant figure is the first non-zero number. Therefore, in this case, the reading may have been 41 ml to two significant figures, but it becomes 40 ml when taken as one significant figure.