To solve this expression, we need to follow the order of operations, which is parentheses, exponents, multiplication and division (from left to right), and addition and subtraction (from left to right).

First, let's simplify the multiplication and division:

25/36:15/12×7/8 = (25/36) ÷ (15/12) × (7/8)

To divide fractions, we can multiply the first fraction by the reciprocal of the second fraction:

= (25/36) × (12/15) × (7/8)

Now, let's simplify the fractions:

= (25 × 12 × 7) / (36 × 15 × 8)

= 2100 / 4320

To simplify this fraction, we can divide both the numerator and denominator by their greatest common divisor, which is 120:

= (2100 ÷ 120) / (4320 ÷ 120)

= 17.5 / 36

Therefore, 25/36:15/12×7/8 simplifies to 17.5/36.