The semi interquartile range seems to be half the difference of the first and third quartiles.

So you'd find out the values of the 0.25 and 0.75 on the CDF of the Normal distribution N(500,100)

I get z(0.25) = -0.6745, that should tell you what z(0.75) is and you can denormalize it in the usual way.

Then just take half the difference of the two.