okay so I'm home now and I've tried what was suggested but, I still don't get the right ans...:
510/1.75=8.30/(pi/4)*d^2
I used the reciprocals and got 1.75/510=(pi/4)*d^2/8.30 ... reduced and I got
sqr root (4*8.30*1.75/pi510)=d ans:.5982 the book ans is 6.02... where did I go wrong?