text: ok here it goes, so first the formula is S(n)=a(1-r^n) all over 1-r\n\nsub\n511/640 = 2/5 * (1-r^9) all over 1-r\nmultiply both sides by 5/2 to make one side variables only\nthat will become\n511/256= 1-r^9 all over 1-r\ncross multiply\n511-511r=256-256r^9\n\narrange it\n256r^9-511r+255=0\n(256r^3-255)(r^3-1)=0\nr= cube root of 1 or just simply one. but it can't be one cause when u sub it it won't be 511/640 neither -1.\nso get the cube root of 255/256\nso the answer would be  cube root of 510 over 8