do tong 5 so nguyen bat ki la 1 SND nen co it nhat 1 SND.Trong 31 so ta lay ra 1 nhom gom 5 SN khi do ton tai it nhat 1 SN,ta tach so nguyen do ra.Nhu vay ton tai 30 SN.ta nhom 5 SN vao 1 nhom nen co so nhom la 30 : 5 =6(nhom)do tong moi nhom la 1 so nguyen duong nen tong 6 nhom la 1 SND hay tong 30 so la SND.ta cong SND tach ra luc dau vao co 31 so la SND