Number of rag dolls made by Rita = 115
Number of buttons Rita uses on each rag doll = 10
Then
Total number of buttons required by Rita = 115 * 10
= 1150
Number of blue buttons used by Rita = (2/5) * 1150
= 2 * 230
= 460
Then
The total number of red buttons used by Rita = 1150 - 460
= 690
So Rita uses 690 red buttons for making 115 rag dolls. I hope the procedure is clear enough for you to understand.