The amount of interest on a given principal is found by ...
I = Prt . . . . principal P earning annual rate r for t years.
We want ...
400 = P×0.09×1
P = 400/0.09 ≈ 4444.44
Alex needs to borrow $4444.44.
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<em>Additional comment</em>
For exactly $400 in interest, the amount is not an integral number of cents, so some rounding of interest or loan amount must occur. In that case, the minimum to borrow is that which will give a calculated interest of at least $399.995. That amount is $399.994/0.09 ≈ $4444.3888.... ≈ $4444.39. Alex can borrow a minimum of $4444.39 and still have to pay $400 in interest.
A is 14 total B is yes because the gained 11 and lost 3 which means they gained 8 after taking away the ones lost and they are still on the positive side so they keep th ball.