300 per month is 7.5% of Jan's earnings
Jan's monthly and yearly earnings
300 = 0.075×(monthly earnings) . . . . . given
300/0.075 = monthly earnings . . . . . . divide by 0.075
4000 = monthly earnings
There are 12 months in a year, so Jan's yearly earnings are 12 times her monthly earnings.
... yearly earnings = 12×monthly earnings = 12×4000 = 48,000
Jan's monthly earnings are 4,000.
Jan's yearly earnings are 48,000.