Help on an algebra problem

<p>what is the common difference when a10-a20=70 in a series?</p>

<p>a<em>10 + d = a</em>11
a<em>10 + 2d = a</em>12
...
a<em>10+10d = a</em>20</p>

<p>so </p>

<p>a<em>10-a</em>20 = -10d
70=-10d
d=-7</p>

<p>wow. i feel so stupid. thanks</p>