This is a linear equation y=ax+b, where b is the starter of 5 pennies and a=3 is the number of pennies per day, and x is the number of days. We use y to tell us how much in pennies she has after x days. So y=3x+5. 1.50 is 150 pennies. To find out how many days it takes to accumulate 150 pennies, put y=150=3x+5. 3x=145, so x=48+... After 48 days she has 1.49 and the next day she has 1.52, so she has at least 1.50 after 49 days. After 10 days she has 3*10+5=35 pennies.