Mr and Mrs Rich opened a joint bank account. On Monday $800 was withdrawn. On Tuesday half of what was left was taken out. On Wednesday they deposited $1650. The account was closed on Friday when $2800 was withdrawn. How much was in the bank account before the first withdrawal?

Start with ending balance, subtract deposits, add checks.

let x: bank account before first withdrawal,
Monday: withdrawn (800), left (x-800)
Tuesday: withdrawn (0.5 * (x-800)), left (0.5 * (x-800))
Wednesday: left (0.5 *(x-800)) + 1650
Friday: account closed and 2800 withdrawn ==> all left was with drawn

0.5 * (x-800) + 1650 = 2800.. simplify and solve for x
x = $3,100.