Question 521220
Let
Present age of Leticia = l
Present age of Kate = k


Leticia is 21 years older than Kate
l=k+21................(1)


In 2 years, 
Leticia's age will be = l + 2
Kate's age will be = k + 2
Then,
Leticia will be twice as old as Kate.  
l + 2 = 2 (k + 2)
l + 2 = 2k + 4
l = 2k + 4 - 2
l = 2k + 2 ..................(2)

Put the value of l from (1) to (2)
k + 21 = 2k + 2
k - 2k = 2 - 21
-k = -19
Multiply by -1 both sides
k = 19
Put the value of k in (1)
l=k+21
l=19+21
l = 40


Present age of Leticia = l =  40 years old
Present age of Kate = k = 19 years old