Let's calculate step by step:

Step 1: Interest-only payments for first 5 years

• Monthly interest rate = 5%/12 = 0.41667%
• Interest-only payment = $100,000 × 0.0041667 = $416.67

Step 2: Remaining balance after 5 years Since it's interest-only for first 5 years, principal remains $100,000

Step 3: Self-amortizing mortgage for remaining 25 years

• Remaining term = 25 years × 12 = 300 months
• Monthly payment for self-amortizing portion: PMT = P × [r(1+r)^n] / [(1+r)^n - 1] = $100,000 × [0.0041667(1.0041667)^300] / [(1.0041667)^300 - 1] = $100,000 × [0.0041667 × 3.48129] / [3.48129 - 1] = $100,000 × [0.014505] / [2.48129] = $100,000 × 0.0058459 = $584.59

Step 4: Principal portion in 61st month The 61st month is the first month of the self-amortizing period.

• Total payment = $584.59
• Interest portion = $100,000 × 0.0041667 = $416.67
• Principal portion = $584.59 - $416.67 = $167.92

Therefore, the principal portion in the 61st month is $167.92.