Problems on H.C.F and L.C.M - General Questions

Find the greatest number that divides 83, 15, 51 leaving the same remainder in each case.

Three bells toll at intervals of 15, 20, 10 minutes. If they toll together now, after how many minutes will they next toll together?

Find the H.C.F. of 115, 138 and 161.

Three bells toll at intervals of 12, 18, 2 minutes. If they toll together now, after how many minutes will they next toll together?

Three bells toll at intervals of 9, 20, 15 minutes. If they toll together now, after how many minutes will they next toll together?