Let x be the the number to be added.
Then, x+1,x+10,x+46 are in G.P.
⇒(x+10)2=(x+1)(x+46)
⇒x2+100+20x=x2+47x+46
⇒27x=54
⇒x=2
Adding 2 to the numbers 1, 10 and 46, we get 1 + 2, 10 + 2, 46 + 2=3, 12, 48, a GP with first term 3 and common ratio 4.