Q30:

From May 1 to May 30 in the same year, the balance in a checking account increased. What was the balance in the checking account on May 30 ?

(1) If, during this period of time, the increase in the balance in the checking account had been 12 percent, then the balance in the account on May 30 would have been $504.

(2) During this period of time, the increase in the balance in the checking account was 8 percent.

(1)如果上升12%, 则balance 为504. 设may1 的时候为 X, 则有方程X*1.12=504 可得X=504/1.12

(2)实际上升8%, 所以may30的确balance=X*8%=(504/1.12)*1.08