If x = 0.72 + 0.8 + 3.87 - 1.089, then number of significant digits in the value of x is______________

$x=0.72+0.8+3.87-1.089=4.301=4.3$

(The number of decimal points in addition or subtraction should equal to minimum number of decimal points of a quantity involved in the operation.) but x= 4.3  ;   x is having two significant figures.

Thus, the number of significant digits is 2.