In a mathematics class, 20 children forgot their rulers and 17 forgot their pencils. Teacher said "Go and borrow them from someone at once”. 24 children left the room, then how many children forgot both:
13
n(R)=20,n(P)=17
n(R∪P)=24
n(R∪P)=n(R)+n(P)−n(R∩P)
n(R∩P)=20+17−24=13