It's an easy-looking grade school equation but can you remember the golden rule to get the answer?

Are your arithmetic skills from grade school still intact? Test them out with this sum that has left internet users baffled.
Nature

Geoscientists vote on Earth’s biggest challenges

From ecosystem change and soil quality to hydrology and natural hazards, Earth studies have never been so important. But in a crowded field of concerns, how can researchers prioritise the most ...