Can You Solve This Equation Using Mental Math 1.7=d/4?

First, recognize that multiplying both sides by 4 will give you
4(1.7) = d

Next, recognize that there is a trick to mental multiplication by 4: Multiply by 2 twice.
2(2(1.7)) = 2(3.4) = 6.8 = d.

Several numbers are easy to multiply in your head:
4 = 2*2
5 = 10/2  (multiply by 10, divide by 2)
8 = 2*2*2
9 = 10 - 1  (multiply by 10, subtract the original number)
10 = move the decimal point one place to the right
20 = 2*10
100 = move the decimal point two places to the right.

15 = (1 + 1/2)10  (add half the number to itself, multiply by 10, or do it in reverse order: Multiply by 10 and add half the result. You can use this for figuring tip amounts in your head: 15% = 15/100, divide by 100, then multiply the result by 15. Another tip about tips: Where I live, a 15% tip is about double the sales tax. Once you learn whatever the relationship is for your local sales tax, the math can be easier than computing 15%.)