I remember during the school days , the teacher asked the class "can you tell me the sum of the ...

I remember during the school days , the teacher asked the class "can you tell me the sum of the first 50 odd numbers ?". I ran quickly to the teacher and told her "the answer is 2500". The teacher replied "lucky guess". She then asked me "can you tell me the sum of first 70 odd numbers ?". I wait for approx 10 seconds and replied with the correct answer. How can i answer so quickly and whats the correct answer ?