Why does a mutation not always lead to a visable change

Sally deposited money into a savings account paying 4% simple interest per year.the first year,she earned $75 in interest.how much interest will she earn during the following year?

Whats is the value of the variables ?

Avideo game enthusiast saved $750 to spend on a video game player and games. the player costs $400. the games costs $49 each. at most how many games can this enthusiast buy along the player