Explanation:

We have,

T  (20) = 2 + 0 = 2 → B

A (1) = 1 + 1 = 1 → A

B (2) = 2 × 2 = 4 → D

L (12) = 1 + 2 = 3 → C

E (5) = 5 × 5 = 25 → Y

P (16) = 1 + 6 = 7 → G

A (1) = 1 × 1 = 1 → A

D (4) = 4 × 4 = 16 → P

S (19) = 1 + 9 = 10 → J

Similarly,

B  (2) = 2 + 2  = 4 → D

A (1) = 1 + 1 = 1 → A

C (3) = 3 × 3 = 9 → I

K (11) = 1 + 1 = 2 → B

S (19) =  1+9  = 10 → J

P (16) = 1 + 6 = 7 → G

A (1) = 1 × 1 = 1 → A

C (3) = 3 × 3 = 9 → I

E (5) = 5 × 5 = 25 → Y

Hence, BACKSPACE will be written as DAIBJGAIY.