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Counting orders in a ramp

Mental mathEasy

A trader sends orders in a ramp: $1$ lot in the first minute, $2$ lots in the next, and so on up to $20$ lots in the last minute. How many lots in total?

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Hint

Pair the smallest term with the largest.

Solution

Pair $1$ with $20$ , $2$ with $19$ , and so on; each pair sums to $21$ .
There are $20 \div 2 = 10$ such pairs.
The total is $10 \times 21$ .
So the total is $210$ .

Answer: $210$

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