In each example of this exercise only the beats are displayed in the measure. Simply count the number of notes to find the number of beats. This will be the top number of the time signature.
The type of note you see in each example will be represented by the bottom number of the time signature: eighth notes should be an 8, quarter notes should be a 4, half notes should be a 2.
*If the time limit is stressful, try Practice Mode.