How many ways are there to put 4 distinguishable balls into 3 indistinguishable boxes?
Because the box is indistinguishable, we can use the reccurence relation. \(B(n,k)=B(n−1,k−1)+kB(n−1,k)\)with \(B(n,1)=1B(n,1)=1\) and \(B(n,n)=1B(n,n)=1\). For your case, you need the sum \(B(4,1)+B(4,2)+B(4,3)\). I trust you can use the formula to complete the problem.