Huh, that’s true of any number that ends in 9.

XY + X + Y = 10*X + Y

Y’s cancel,

XY = 9X => Y = 9 for any non-zero finite value of X.

so for 69? X = 6, Y=9

(6*9) + 6 + 9 = 10*6 + 9

54 + 15 = 69

69 = 69 (nice!)

429? X = 42 Y = 9

(42*9) + 42 + 9 =10*42 + 9

(378) + 51 = 429

429 = 429

Even if 10X+Y doesn’t equal something that ends in 9 it works

X=3.14 Y=9

(3.14*9) + 3.14 + 9 = 10*3.14 + 9

28.26 + 12.14 = 40.4

40.4 = 40.4

Doesn’t work if Y =\= 9:

68? X = 6 Y = 8

(6*8) + 6 + 8 ?= 10*6 + 8

(48) + 14 ?= 68

62 =\= 68