In a group of 120 pupils, 80 can play guitar only and 25 can play piano only. How many pupils can play both instruments if there are 3 pupils who can play neither?

Answer 1

I bet you are learning about Venn diagrams

Draw 2 intersecting circles within a rectangle, and label them G and P
place x in the intersection of both circles
place 3 in the region inside the rectangle but outside of the circles.
Place 80 in the G circle not intersected by P
Place 25 in the P circle not intersected by G

solve:
x + 80 + 25 + 3 = 120 , and the mystery is solved.