A +10 = 3(B - 10)
⇒ A - 3B = -20 ....(i)
And ( A - 10) = (B + 10)
⇒ A - B = 20 ...(ii)
on subtracting Eq.(ii) from Eq.(i), we get
(A - 3B) - (A - B) = -20 - 20
⇒ - 2B = - 40
there4; B = 20
Now, by using Eq. ...(ii),
there4; Ratio of the numbers of students of A and B = 40 : 20 = 2 : 1