Alex spent 1/3 of his pocket money on a toy airplane and 2/3 of the remainder on a toy robot. He had $20 left. How much did he spend altogether?

He spent $70 in total! He spent ⅓ of his money on a toy plane remainder ³/³ -

⅓ = ⅔. Spent on toy robot = 2/3 * 2/3 = 4/9

Total spent = 1/3 + 4/9 = 3/9 + 4/9 = 7/9

Left = 9/9 - 7/9 = 2/9

Then the final step,

2/9 to $20

1/9 to 20/2 = $10

7/9 to 10*9 = $70