In two box 123 coins. Ten coins from a first box shifted to the second box. The second box now having twice as many coins in the first box. How many coin in the first box before the shift?
original number in first box --- x
original number in 2nd box --- 123-x
after shift:
first box ----- x-10
2nd box --- 123-x + 10 = 133-x
133-x = 2(x-10)
solve for x