How does interest work on apple barclay credit card?

If I purchase the iPhone XR for 749 dollars, the interest rate is 28.99 percent. So how is that interest applied? If it's applied monthly how does that work? I would be paying 100 dollars a month toward the 749 so with that interest rate how long would it take me to pay it off?

It will take 9 months. The total interest will be $87.

Interest on all bank accounts and credit cards are calculated daily on the outstanding balance. (on that day).
Peace.

Interest works the same, no matter what. The annual rate of 28.99% (which is obscene, but whatever) is compounded 12 times every year

1 + 0.2899/12 =>
12.2899/12 =>
r

749 * r - 100) * r - 100) * r - 100) * … ) * r - R = 0
749 = 100/r + 100/r^2 + 100/r^3 + … + 100/r^n + R/r^n

1/r = k

749 = 100k + 100k^2 + 100k^3 + … + 100k^n + Rk^n

S = 100k + 100k^2 + 100k^3 + … + 100k^n
Sk = 100k^2 + 100k^3 + … + 100k^n + 100k^(n + 1)
Sk - S = 100k^(n + 1) + 100k^n - 100k^n + … + 100k^2 - 100k^2 - 100k
S * (k - 1) = 100k^(n + 1) - 100k
S * (k - 1) = 100 * k * (k^n - 1)
S = 100 * k * (k^n - 1) / (k - 1)

749 = 100k + 100k^2 + 100k^3 + … + 100k^n + Rk^n
749 = 100 * k * (k^n - 1) / (k - 1) + R * k^n

k = 1/r = 1 / (12.2899/12) = 12/12.2899

749 = 100 * (12/12.2899) * ((12/12.2899)^n - 1) / (12/12.2899 - 1) + R * (12/12.2899)^n
749 = 100 * (12/12.2899) * (1 - (12/12.2899)^n) / ((12.2899 - 12)/12.2899) + R * (12/12.2899)^n
749 = 100 * 12 * (1 - (12/12.2899)^n) / (0.2899) + R * (12/12.2899)^n
749 = 1200 * 10000 * (1 - (12/12.2899)^n) / 2899 + R * (12/12.2899)^n
749 = 12 * 10^6 / 2899 - (12 * 10^6 / 2899) * (12/12.2899)^n + R * (12/12.2899)^n
749 - 12 * 10^6 / 2899 = (R - 12 * 10^6 / 2899) * (12/12.2899)^n
(749 * 2899 - 12000000) / 2899 = (R - 12000000/2899) * (12/12.2899)^n
749 * 2899 - 12000000 = (2899R - 12000000) * (12/12.2899)^n

(750 - 1) * (2900 - 1) - 12000000 =>
75 * 29 * 1000 - 2900 - 750 + 1 - 12,000,000 =>
75 * (30 - 1) * 1000 - 3650 + 1 - 12,000,000 =>
(2250 - 75) * 1000 - 3649 - 12,000,000 =>
2,175,000 - 3649 - 12,000,000 =>
175,000 - 3649 - 10,000,000 =>
171,351 - 10,000,000 =>
-(10,000,000 - 171,351) =>
-(9,828,649) =>
-9828649

749 * 2899 - 12000000 = (2899R - 12000000) * (12/12.2899)^n
-9828649 / (2899 * R - 12,000,000) = (12/12.2899)^n
9828649 / (12,000,000 - 2899 * R) = (12/12.2899)^n

If R = 0

9828649/12000000 = (120000/122899)^n
ln(9826849/12000000) = n * ln(120000/122899)
n = ln(9826849/12000000) / ln(120000/122899)
n = 8.369451184901753911783484791365

So you're going to make 8 full payments of 100 and then a 9th partial payment

749 * 2899 - 12000000 = (2899R - 12000000) * (12/12.2899)^n
-9828649 = (2899 * R - 12,000,000) * (12/12.2899)^8
9828649 = (12,000,000 - 2899 * R) * (12/12.2899)^8
9828649 * (12.2899/12)^8 = 12,000,000 - 2899 * R
2899 * R = 12,000,000 - 9828649 * (12.2899/12)^8
R = (1/2899) * (12000000 - 9828649 * (12.2899/12)^8)
R = 35.593851611203810762004981078606

8 full payments of $100 and a final payment of $35.59 for a total of $835.59

835.59 - 749 => 86.59

You'll pay $86.59 in interest.

The other answers showed you the quick and easy way (using a formula or a spreadsheet in Excel). I'm just showing you the math behind the formula. This stuff doesn't just materialize. There's a method to the madness.

Don't ever pay that much interest. Save and pay cash.

Interest rate is annual, so divide by 12 to get the monthly amount. 28.99% APR = 2.42% monthly.

So if you pay nothing when you walk out with the phone, after 1 month you owe the $749 plus about $18 of interest.
You pay your $100, bringing your debt down to $667.
Because your balance is lower, the next month you only add $16 of interest.

It should take about 9 months to pay off the phone.

Or you could save up your money for 8 months and then buy the phone without paying interest.

Note that $749 is probably the before-tax price on the phone, you'll add a protective case and/or insurance, and you'll also be paying for a data plan at the same time.