Harry owes the bank money. To repay his debt, he paid $150 back to the bank each month. After 10 months, his remaining debt was $6900
What was the original amount of Harry's debt?
How long did it take Harry to pay back his entire debt?
Let's assume the original amount of Harry's debt is x.
After 10 months, he has repaid $150 per month for a total of 10 * 150 = $<<10*150=1500>>1500.
Therefore, his remaining debt is x - 1500 = $6900.
Subtracting $6900 from both sides of the equation, we get x = $6900 + $1500 = $<<6900+1500=8400>>8400.
Therefore, the original amount of Harry's debt was $8400.
To find out how long it took Harry to repay his entire debt, we divide the remaining debt by the monthly payment: $8400 / $150 = <<8400/150=56>>56 months.
Therefore, it took Harry 56 months to pay back his entire debt. Answer: \boxed{56 \text{ months}}.