Brain Teasers and Puzzles 

Workings

Hint
Some are much more common than others.

Answers
Common: vacuum, vacuumed, vacuuming, vacuums.

Less common: continuum, continuums, muumuu, muumuus.

Much less common: carduus, carduuses, continuua, duumvir, duumviral, duumvirate, duumvirates, duumviri, duumvirs, individuum, individuums, lituus, lituuses, menstruum, menstruums, mutuum, mutuums, paramenstruum, paramenstruums, residuum, residuums, squush, squushed, squushes, squushier, squushiest, squushing, squushy, triduum, triduums, ultravacuum, ultravacuums, weltanschauung, weltanschauungen, weltanschauungs.

Workings

Hint
The first word ends in k.

Answer
Look before you leap.

Workings

Hint
Imagine a set of scales.

Answer
They are all the same!

Workings

Hint
The cube root of the given number is a good starting point.

Answer
71, 73, 79.

Reasoning
The cube root of 409,457 is around 74.

The closest consecutive prime numbers to 74 are: 61, 67, 71, 73, 79, 83, 89, 97.

Using a calculator, it is quickly seen that the answer can only be found using 71, 73 and 79.