Kit question on Investment appraisal.

It might seem silly but i didnt follow the logic of IRR in KIT(BBP) question “slow fashions part a”.
I mean how is he using the irr, the decision rule in selection of the projects in IRR if i am not wrong is that is that IRR must be higher than cost of capital, but in the question combine IRR of all projects is calculated, which is confusing me.