choice of discount factor in IRR

Hello I have something to ask in regard of the choice of the discount rate when calculating the IRR. For example in question 1 from December the discount rate is 11%: the pv of the project is positive so we choose an higher discount rate to calculate the second npv. I used 16% however when calculating the npv it was positive instead of negative. then I used 17% and I obtained a negative npv. IN the exam using 16% would have been time consuming: is there a way of deciding which is the best way of deciding which is the second discount rate to use? Many thanks Roberto