Keiko has a total of $5200
x is the amount of money in larger account
y is the amount of money in smaller account
x - y = $900
and x + y = $5200
This creates two simultaneous equations:
x - y = $900 ... (i)
x + y = $5200 ... (ii)
Adding (i) and (ii) :
2x = $6100 , x = $3050
y = x - $900 = $3050 - $900 = $2150
The amount of money in larger account (x) = $3050
The amount of money in smaller account (y) = $2150