In 40 days of menstrual cycle the ovulation occurs at :

26th day Ovulation is estimated to occur 14 days before the first day of succeeding cycle and this interval is fixed. In case of irregular cycles, it is the follicular phase which varies, but the luteal phase remains more or less constant at 14 days. therefore day of ovulation can be counted by counting 14 days backward. As in the question a cycle of 40 days, ovulation will occur 14 days prior to next menstruation i.e. (40-14) = 26 day