Question 1035975
{{{a[n]=a[1]+d(n-1)}}}

Look at this carefully, it's the secret to solving arithmatic sequence equations. It simply states that the nth term is the first term plus n-1 times the difference between terms. Why n-1? Because for the first term, you don't add any difference. And for the second term you add just one, etc. 

For your problem. 

{{{a[10]=250+30(10-1)}}}

{{{a[10]=250+270}}}

{{{a[10]=520}}}

not time consuming.