mongo db querying array values with greater and less than

Use $elemMatch to constrain both parts of the range query to the same marks element:

db.users.count({
    marks: {$elemMatch: {$gte: 20, $lt: 30}}
})

Isn't it just this:

db.scores.count( { 'marks' : { '$gt' : min_value , '$lt' : max_value } } )

Which in your case would translate to:

db.scores.count( { 'marks' : { '$gt' : 20 , '$lt' : 30 } } )

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