David had $80. He spent 4/5 of his money on books. How much did he have left?

Question

Tape diagram to solve answer

 

Answer 1

spent 4/5 of the money, so one fifth of the money is left.

one fifth of $80 = $16 since 5 times 16 is 80.

Alternatively, 1/5 = 0.2 so 0.2 times 80 is $16.

Dr. Gulshan is right, of course!

 

Also, one fifth is two tenths; one tenth of $80 is $8, so two tenths must be twice $8.  Example of mental math.

Answer 2

Spent =80*4/5 = 64

80- 64 = 16