i have the question

'find the sum of the artithmetic pregession 1, 4, 7, 10, ........ 1000'

and then 'every third term of the progression is removed i.e 7 and 16. the the sum of the remaining terms'

i know the formulas for the nth term and the the sum of the terms, but i cant find a way of finding out the number of terms in the sequence which i thnk will then allow me to simply put the number in the formula.

but then again i have no idea how to even start the 2nd part.

please help ive been doing this for hours and theres still another 2 questions i cant do, thanks