Question

The first one how to do it I’m confused

Answer 1

She has $900 total in her account.

she needs to get the laptop, that's $600 in costs.

and she pulls out $60 for food, how many times can she do that?

the first time she pulls them out, the amount is 60,

the second time she does it, the amount will have been 60 + 60 or  60(2),

the third time she does it again, the amount withdrawn will be 60(3)

the fourth time of the withdrawal the amount pulled out will be 60(4)

now, if she does "f" times for food, the "f" time she does it, the amount withdrawn would be 60(f) or just 60f.

now, we know the laptop costs $600, and her food withdrawals are 60f, she's not putting anything in the account, so the 900 bucks are just shrinking with every withdrawal for either laptop or food.

so, her expenses are, the laptop and food, 600 + 60f.

whatever "f" may be, that amount must never be greater than 900, because that's all she has in the account, and if she uses more the bank will let the dobermans loose on her.

so, 600 + 60f must be "less than or equals to" 900.

900 ⩾ 600 + 60f

now, how many times can she withdraw?   what's "f"?

$\bf 900 \ge 600 + 60f\implies 300\ge 60f\implies \cfrac{300}{60}\ge f\implies 5\ge f$