sam gave 2/5 of his money to his wife and spend 1/2 of the remainder. If he had #300 left, how much money did he have at first?
Let the total be T
$$\\\frac{3}{10}\;of\;T=300\\\frac{1}{10}\;of\;T=100\\All\;of\;total=1000\\$$
If he originally had M then if he gave (2/5)M to his wife he would have (3/5)M left. If he spent half of this he would have half of it left. Half of (3/5)M is (3/10)M. This is equal to #300, so
$$\frac{3}{10}M=#300\\M=\frac{10}{3}\times #300\\ M=#1000$$
