The number of ways in which the letters of the word ARRANGE can be arranged such that both R do not come together is
900
The word ARRANGE, has AA,RR, NGE letters. That is two A' s, two R's and N, G, E one each.
∴The total number of arrangements 7!2!2!1!1!1!=1260
But, the number of arrangements in which both RR are together as one unit = 6!2!1!1!1!1! = 360
∴The number of arrangements in which both RR do not come together = 1260 - 360 = 900.