Check the picture below.
when dividing decimals, we first off, count how many decimals each side has, and add as many zeros to the other.
in this case 3.5 has one decimal, so the 7 gets padded with a one zero, to become 70, once we do that, we lose the "dot" on 3.5, so we're dividing 35 ÷ 70.
well, we can't use 1, so we use the lower value below that, namely 0, we get a remainder of 35, we can't pad it with anything, so we add a decimal dot to the quotient, thus allowing each subsequent remainder to be padded with a 0, we end up with 350 ÷ 70 and you can see the rest.