In 4 weeks, Mark earns an average of $400 per week. If, after working another 2 weeks at a constant salary, his average earnings per week are $500, how much did he earn in the last 3 weeks

Ok ... so week #4 he earned $100 approximately. this is so because in the four weeks of working, he made in total, $400. so 400/4 = 100.

For weeks #s 5 and 6, he made a total of $1,000 because he made 500 for week 5 and 500 for week six (the last 2 weeks) ...

So all in all, he made $1,100 in the last 3 weeks ...

Do you get it?

The answer is 1400 because he got 400$ on the third to last week, 500$ on the second to last week, and 500$ on the last week. that adds up to 1400$