Hi eddy. Start experimenting with numbers. (The goal is to form an equation to solve.)

We don't need a symbol for the number of robots. We can express the number of robots in terms of the number of dolls.

Let symbol d represent the number of dolls.

Then the expression 300-d represents the number of robots.

If there were 299 dolls, would the remaining money equal the value of 1 robot?

If there were 298 dolls, would the remaining money equal the value of 2 robots?

If there were 297 dolls, would the remaining money equal the value of 3 robots?

Write out the the arithmetic, for answering those three questions. Do you see that you're doing the same operations?

Now think: if there were d dolls, would the remaining money equal the value of 300-d robots?

Use the pattern of your calculations to try form an equation. If you get stuck, please show us what you tried. Thanks!