See why decimal multiplication puts the decimal point exactly where it does, using a 100-square grid, and explore how to turn decimal division into whole-number division.
When multiplying two decimals with one digit after the decimal point, multiply them like whole numbers, then shift the decimal point left by the sum of their decimal places. For example, 0.3 × 0.4: work out 3×4=12, then shift by 2 decimal places (1+1), giving 0.12. Drawing this on a grid of 10 columns by 10 rows, 100 squares in total, shows exactly why.
Decimal division gets confusing when the divisor is a decimal. So multiply both the divisor and the dividend by 10 to turn the divisor into a whole number, then calculate. For example, 1.2 ÷ 0.3 can be solved by multiplying both sides by 10, turning it into 12 ÷ 3 = 4.