TASK DEFINITION: Given a sentence, fill out the missing word with a 'no' or a number (between zero and ten). You should write the numbers with english alphabet, like: four instead of 4.
PROBLEM: Natural ytterbium is a mixture of ____ stable isotopes, with seven other unstable isotopes known.

SOLUTION: seven

PROBLEM: Marsupials have ____ vaginas and a double uterus.

SOLUTION: two

PROBLEM: Healthy eating is based on ____ simple principles -balance, moderation and variety.

SOLUTION:
three