When we know a specific amount of something, we simply use a number to express it: 'During the first phase of the project, 90 per cent of patients said they felt better than prior to the treatment.' However, sometimes we only have a vague idea of the amount and are not sure of the exact quantity. Then we need the help of other words such as 'some', 'about' and 'around'. 'It involves the treatment with acupuncture of some 500,000 people with chronic back pain and headache.' 'About 40,000 doctors and 5,000 health practitioners in the country now practise Chinese medicine, according to the head of the German Acupuncture school in Dusseldorf.' We may also use '-plus' as in 'He has 20-plus years of experience in the field.' It means he has between 21 and 29 years of experience, but the writer is not sure. If you're not sure about the exact year in which something took place, you can simply say 'in the 1970s' (i.e. 1971 to 1979). Note that there's no apostrophe, i.e., not 1970's.
